TM15 and the TM7 are both mini internals. I have a TM15 my roommate has TM7. The big difference is the style of the shell/housing. TM15 is a magnesium composit while the tm7 is a plastic polymer. TM7 is lighter more compact but is also imo more fragile as the housing in the grip area is weak and prone to damage if you run tank on marker. The TM15 doesn't have this issue as the magnesium shell is more rigid and robust.
But I will agree TM15 or TM7 will perform better. However they need to be properly maintained. Where the bt-4 you can pretty much never maintain but wont shoot as fast or as smooth as a TM series marker.
I used to run a TM-15, shoot beautifully right out of the box. I don't think I ever broke a ball in the breech through numerous cases of paint. Its super quiet too because of how the shroud and barrel are designed.
I used to own a TM15. It was a good gun, very quite, decent consistency. Tear down takes a little time but you dont need to do that often. It is a little bulky but you can cut down the RIS shroud to a shorter version and use a 8 or 10 inch barrel to help with length. Id go with the TM15 or TM7 over the BT's just due to how much more consistent they will be. My friend has a TM-7 and it has issues sometimes with the solenoid not working. Havnt seen that with the 15's. Or you could always rock an A-5 for less money. They can be upraded easily tuned for better performance, just use hpa.
Probably the M-17 for magfed considering all factors like price, performance, company support etc. I hear the rap4 468 is pretty good expecially with the scarab mags. I'd go with one of the two. But if you want to wait a couple months the T-15 might finally arrive. Cant say it promises to be better than the M-17 but the final version may have a leg up in an area or two.