There are lots of aftermarket... larger than stock turbos... that still are "stock appearing"...... at this point... you will probably have to take the turbo off... and remove the compressor housing and take a couple of measurements... the compressor wheel is almost cone shaped... you have to measure the big end and the little end of the cone.... the big end is the inducer... the little end is the exducer.... then you can determine what size the compressor side is...
The exhaust side measures like the compressor side.... except the terms are backwards... the little end is the inducer... the big end is the exducer....
You can probably determine the size based on these measurements...
There might be some slight variations... but if you have the correct measurements... and some pictures.... there are some people on here than can probably tell you exactly what you have.... and consequently... the HP potential...