Your tuner shouldn't be pushing anything with whatever you've asked for, he should be monitoring temps, knock, fuelling etc etc, and set everything at the correct levels for the highest gain, but keeping it within normal operating limits.
Mark Dance (Pulley) achieved great gains with his custom free flowing setup, but you'll never see anywhere near the same power on stock twins, as you will with a decent single setup.
That being said, the way the twins deliver power is not to be sniffed at. I actually miss mine at bpu sometimes, as the car was a pleasure to drive. I went single to simplify everything, as I knew that 20 year old components would eventually fail.
You've also got to factor in the auto transmission. If you want to push past bpu, I would strongly advise you shim the box, run an additional cooler with a thermostatic switch, and do a full flush and fluid change.