Maybe its an ultra low reading dyno, either that or something is seriously holding it back. An Atr45sat should be easily over 300rwkw on e85 even on 16psi. A member on here had that turbo on a 33 and it made 290rwkw on 98, around 18psi on jem's dyno.
My own car has a HG 21u high flow. Made 260 rwkw on 98 18psi and 312 rwkw on e85 19psi and that's way smaller than an Atr45sat. Stock manifold, internal gate, return flow cooler, holds boost pretty good up top and iat is pretty good. Figures are on a hub dyno.
Car has done at least 25 track days over the last few years, stock engine has held together well, compression is still around the 160psi mark. I recently had the head off for a suspected head gasket issue that was a wrong assumption. 23 year old gasket was still in perfect condition. Obviously i replaced it since i was there with a new oem gasket, serviced the head and added arp head studs. Didn't really need any of that.
I reckon 300 rwkw on e85 is a happy place for a healthy rb25. As said above look at your data and try to pinpoint anything that doesn't look right.
At the end of the day, power is not the only thing that makes a car quick. Setting up the car to turn, brake, grip and have a nice useable power delivery is better than a billion kilowatts. Ive chopped cars with way more power than mine at the track and have also been chopped by cars that have way less power. Lol.