Seeing the work on socs come in already is absolutely fantastic, especially the Gravity Garden badniks.
Here is the link to a prototype final or near-final boss that Whackjood requested I post here.
http://www.sendspace.com/file/rgazbv
Although, I need to pick apart the boss you submitted 742mph. I adore the initative you took in making it, but in practice it's not a very good boss. Firstly, it's extremely unclear how to damage it, while there's a reliable method for doing so you do a very poor job at conveying it. The player needs more feedback in game to know that dodging the bosses attacks by staying as far away as possible isn't the correct way of dealing with things.
Also, because the player is encouraged to stay near the boss to do damage, the player needs warning as to what attack is coming up next before it begins. This is fine when the boss is going to fire the THZ turret because of the audio cue from the charging noise, but the other attacks come out without warning.
The pinch phase is unreasonable and cheap rather than difficult as well. The attack where you are shrunk is fine imo, but the combination of rapid fire missiles and THZ turret spam in the second attack is absurd. Also, the method used to damage the boss during the pinch phase is too difficult, even for what is intended to be a final boss.
This is a nice start, and it's more misguided rather than outright bad. If you fixed all of these problems, it would have a place late game, but I doubt I would include it as the final boss of the entire pack.
However, on the topic of final bosses. Would anyone take objection to the final boss taking place in a single act zone after Steam Engine? As a hypothetical example, the player defeats the Steam Engine Zone boss then goes to a new final level to fight the final boss? Similar to the Wing Fortress to Death Egg transition in Sonic 2.