HT18s-2s turbo
 

So I have this Rx7 turbo, it's a HT18s-2s. Compressor side is 63mm and the turbine side is 64mm, and it's oil and coolant cooled. I'm currently running a T3 turbo(specs unknown it came on the car), but I was wondering if it'd be a decent upgrade if I put it on my D15b.

How bad would the boost lag be since the turbine housing looks significantly bigger then the T3's turbine housing? I know the turbo is basically comparable to a T4 from what I've read, but I know everyone says a T3/T4 hybrid is usually as large as you should go. Any help would be great. Thanks!