The difference between the T25 and T28 has to do with the compressor. The T28 simply use a T3 compressor mated to a T25 turbine. Basically the same as Garrett going with a T3T4 which is a T3 turbine and T4 compressor. Its essentially a hybrid turbo. The T25 are smaller and less capable then the T28 though with a limit on Compressor flow to about 30 lb min which is on the high side of them.

It depends on the turbo compressor specs as to what type of HP it will produce. To give you an idea the Nissan SR20DET turbo from the S13 is essentially a GT2554R minus the ball bearing cart. It can make about 280rwhp completely maxed out.

Something else to note is some of the Garrett T25 turbos used plastic (or ceramic) turbines and they are great for spool but can not andle boost. The turbine will literally come apart at anything past stock boost pressure.

I would bet the turbo you are reffering to is the Nissan S13 turbo I mentioned since it uses the .80 housing. It should be snap ring back compressor housing with a 2 bolt inlet and 3 bolt outlet.