playerunknowns-battlegrounds
Watching the PUBG invitational, I realise that the island where the lobby is (the minute before you get in the plane) is still on the map.
While in the match, can you return to that lobby island, and is there any loot there?
Can you go and get the test weapons for instance? I heard that some pretty rare weapons like the tommy gun can be found on the island while waiting, would they still be there?
There may just be coincidences with a lot of people having similar gear. I often see the same stuff because people aim to get the better gear and obviously level 3 gear will be the most benefit to you. The places with higher loot spawns, and better gear tend to have a high percentage of players dropping at so it's not unlikely you'll see a lot of players with that better gear.
As far as I known in my time playing, and from reading forums and threads about loot spawning, there doesn't seem to be any particular weight applied to certain things spawning in different matches.
Common loadouts are ARs with a shotgun or SMG, in my experience, which is why they're so commonly seen. The more powerful guns have less frequency so that also contributes to seeing more SMGs around the place.
Some early threads like these all mention how it's weighted towards certain zones having better loot, and the farther away you are from them, the more RNG is factoring in. In my experience the more isolated places tend to have some things in them, and a variety of gear, but there is the chance nothing/little amounts of things are there.
If you check out various loot spawn guides like this or this, they'll tell you that you're guaranteed better things at places like the Military Base, the School, or the Power Plant to name a few.
Here is an image which shows the general distribution, and as you can see, some places are much more weighted in the amount/quality of loot spawned. 
Is the quality of the weapons, attachments, and gear you find affected by the area that you find it in?
Yes. But there is no real information available about how the loot spawn system works. As the spawning is made by the server so there will be no data mining data.
But by experience it is quite obvious. Especially in squad games you will be able often able to loot areas without competition (in squad games the players are not that much scattered all around the map). ShatterNL map is quite good but not percent. For example in Yasnaya Polyana the loot on the east (hospital and police station) is significant better than the loot in the north-west part of Yasnaya Polyana
Best Answer
Yes you can return to spawn Island, No the weapons on the tables will not be there.
If you have a lucky plane ride and are able to drop on the north east coast of the Main Island, and you are able to find a boat, there is usually enough time to ride over to Spawn Island and return before the circle shrinks.
In terms of what sort of loot you can expect to find, all of the "testing weapons" that are placed during the setting up game-phase are removed before loot is spawned so no chance of finding crate only weapons just lying around. The island does have a decent amount of loot (usually enough for Duo's but a full squad will probably find it lacking) with a decent chance of finding military grade gear.See Map Below