Does that mean that the theoretical maximum stats of characters on the Standard Sphere Grid would be greater than the theoretical maximum stats of characters on the Expert Sphere Grid?
Yes, by ~70 spheres, if I recall correctly.
If that is the case, then what would be the reason for playing using the Expert Sphere Grid?
In the Expert Sphere Grid, all your characters start out very near the center, allowing you to choose different paths fairly early on, rather than having set pathways that you'd need high-level Key Spheres to break out of.
Final Fantasy Wiki
Speed (すばやさ, Subayasa?), also known as Dexterity, Agility, or Agl, is a recurring statistic from the series. It determines how often a character will attack. In games with Active Time Battle or Conditional Turn-Based Battle, it is used to determine the rate at which the ATB or Charge Time gauge rises. It can be augmented or reduced by Time Magic spells Haste or Slow.
Agility determines how often a character gets a turn. Agility can be
increased by activating agility nodes on the Sphere Grid. Rikku has
the best Agility on her path on the grid, Auron and Lulu are the
worst, but eventually, any character can traverse any path. Agility
Spheres can be farmed from the Fenrir on the Monster Arena to further
boost Agility.
The benefit of raising agility caps off before reaching 255. The very
first turn of battle may, from a technical point of view, come
slightly quicker with higher agility, up as far as 250; though this is
negated in the event of a Preemptive Strike or use of a First Strike
weapon. Apart from this first turn, the effect of raising agility caps
off at 170. This first turn effect is virtually negligible, therefore
many players choose to stop raising Agility once they reach 170. It is
worth noting that the Standard Sphere Grid in its default
configuration already contains enough Agility nodes to reach 170.
Aeons also have their own stats separate from the characters'. They
are mainly based on Yuna's stats, but also can be boosted manually and
increase when the player fights more battles. Shiva is the fastest
Aeon, and Ixion the slowest.
GameFAQs stat mechanics (take your time to read everything)
Update: Here comes the part you're interested in.
First, you must be introduced to what Tick Speed is:
From Terence's mechanics data:
"The CTB system works on a system of clock ticks, very similar to FFT.
There are, of course, differences.... Each monster and party member
have a counter. Every tick, this counter reduces by 1. At that point,
anyone whose counter has dropped to 0 may take their next turn. It is
not currently known exactly how the game resolves multiple characters
reaching 0 at the same time. In addition to the counter, every battle
object has a tick speed. This is an integer worked out from the
character's Agility. Any action you take will be a multiple of this
tick speed. When a turn is taken, it sets the counter to a value
related to your tick speed and the ability used. Each ability has a
Rank that designates exactly how fast the ability is. Lower ranked
abilities take less time than higher ranked abilities, and the
progression is linear: a Rank 8 ability will take eight times as long
to recover from as opposed to a Rank 1 ability. The CTB window shows
the next 16 turns that will be taken, in whatever order. It assumes
that each character, when their next turn comes up, will select a Rank
3 move. Rank 3 is the default speed for any ability. Furthermore, the
bar next to each character's 'turn' represents how many ticks away it
is. With no bar showing, the turn is to be taken during the current
tick. A full bright pink bar represents a turn 20 ticks away, and for
each further color that is filled, another 20 ticks is added on. A
full bar of the darkest color represents a turn that is 60 or more
ticks away. Because of all this, there are very few formulas required
to explain all this. We'll cover the most important one now: what the
counter is set to when you take a turn. The exact value is: Counter =
[Tick Speed * Rank * Haste Status] ...where Rank refers to the Rank of the
ability used, and Haste Status is either 1/2 for Haste, 1 for Normal
and 2 for Slow. Keep in mind that when under Haste status, your
Counter will be rounded up if a fraction remains. This often results
in abilities with odd-numbered Ranks used by characters with
odd-numbered tick speeds being recovered from slightly faster than
usual. Casting Haste and Slow also changes the current Counter. A
successful casting of Haste on a character will halve their current
Counter while rounding down, regardless of whether they were Slowed or
not beforehand. The opposite happens with Slow; their Counter will be
doubled regardless of whether they were in Haste or not. Also note
that Dispelling either Haste or Slow will NOT change their Counter.
Their Counter will continue to descend at the same high or low value
until their next turn."
Agi to Agi = Tick speed
170 to 255 = 3
98 to 169 = 4
62 to 97 = 5
44 to 61 = 6
35 to 43 = 7
29 to 34 = 8
......
Best Answer
I believe this is the answer. According to the Sphere Grid article on the same wiki, there are 85 unremovable nodes on either the Standard or Expert Grid, but the Standard Grid contains 775 other nodes you can manipulate or replace, while the Expert Grid contains only 707. That means you'll have 68 fewer nodes you can use to increase your stats.