not as effective as it could have been
This is referring to an event that occurred in the past. An event happened and its effectiveness was sup-par. For example:
Napoleon's efforts at Waterloo were not as effective as they could have been.
As for your other phrase:
not as effective as it could be
This contains the possibility of future effectiveness. A demonstration of the effectiveness has obviously been conducted, and the speaker has decided that improvements in the process could be made so that it is more effective in the future. For example:
The bug spray isn't as effective as it could be. I'm sure the research lab can make it kill bugs more quickly.
(I think I'm going to open a can of worms with this answer but I've done some research so, don't blame me.)
In the student's text book, New English File Upper-Intermediate Oxford University press
Page 138 it says:
The opposite of "must have" is "can't have" NOT "mustn't have"
So for some it is considered standard English to use: can't have or couldn't have instead of mustn't have when you are speculating or guessing about the past in questions and negative sentences.
He couldn't have been hungry
means practically the same as
He can't have been hungry
They both express a strong conviction in the past, the speaker can choose to add further information in order to back up his claim.
A: John didn't eat his cereal this morning.
B: He can't/couldn't have been hungry. He usually has breakfast.
Thus the speaker is saying it's impossible that John was hungry because he knows John never leaves home without eating something. Must not (mustn't) means something quite different, you are forbidding someone or something from performing an action now, in the present and it is not used for speculating in the past.
On p394 in Practical English Usage by Michael Swan:
Must is used with the perfect infinitive for deductions about the past.
- "The lights have gone out" -- "A fuse must have blown."
- "We went to Majorca." -- "That must have been nice."
Must is only used in this way in affirmative sentences. In questions and negatives, we use can and can't instead.
This is also confirmed by A Practical English Grammar by A.J.Thomson A.V. Martinet 4th edition on page 148.
Best Answer
In each pair, the first phrase is about having accomplished (or not) something in the past:
The second phrase in each pair is about opportunity, most typically that was missed: