The 18RG is a nice motor and a great replacement over the 18R in RA23/28/40 model Celica's. In standard spec the RG is rated at 130 to 145 bhp depending on the version, and with your spec's i would guess you would be somewhere near the top end of that range. It's a good increase above the standard 88bhp that an 18R makes. But I agree that a 1G GTE/GZE would be a bigger improvement.
Cheers,
Mitch.