400 whp is fine on a 5MT, just don't shock load it during shifts. Short block will be very iffy to run at 400whp. under 350 whp and you're fine.
The 18g is a fun turbo. I've heard bad things about the AMR turbos, but I can't really vouch for or against.
FYI 350whp is a fast car. The LGT, being turbo, doesn't slam you back in the seat like a V8 does, rather it's a very smooth power delivery.