I don't know much about turbos but I think I can help you out a little. The GT that you here about is a new wheel design that comes in P, Q, R, and S trims. These new wheels are supposed to have better designs that allow for better tradeoffs between power and lag. P gives the least lag, then Q and for larger turbos there are R and finnally S.
As for SP and Turbonetics and all these other companies, they are just who are making or selling the turbos. I think Garret, Turbonetics, Precision and ITS are the biggest manufacturers of the turbos. Places like PHR, BL, SP and everyone else just slap there names on them and sometimes modify them a bit. I think SP, PHR and BL all use Precision turbos in their kits.
BBs are different types of bearings used inside of the turbo. They are supposed to provide better spool and better midrange although people argue about whether it is true or not. i don't think BB turbos are rebuildable but I could be wrong.
The reason that the 67 is bigger than the T78 is because the numbers in their names are not representing the same things. In the PTE67, that 67 represents the size of the compressor wheel. The 78 in T78 doesn't, in fact I don't know what it stands for. I think the T78 has a compressor size of 66mm. The same thing goes for the GT42 which has a very large compressor wheel.
The best way to compare turbos is to compare their compressor/exhaust wheels and exhaust housings. Then you will also be able to see the differences that the newer wheels make when you compare an older style turbo with a newer style turbo that is the same size. Hope this helps, and please anyone feel free to jump in if I left something out or said something that wasn't quite right.
