I don't honestly think there'd be 30hp difference. Considering XBWGN's running a weiand and has 252rwkw (close enough to 350rwhp). I reckon there would be lucky to be 5-10kw at the most @ the wheels. I'm guessing that 30hp you stated is at the engine though?
I'll find out eventually as I'll be running a CHI manifold and I've been told "yes, it does fit under the bonnet of XA-C's". I'll find out for myself and go from there. I'll also see if there is any noticable power difference.
I'm running the 218cc as the 185cc didn't suit my needs. 185cc would have been too restrictive. They would have needed more $ put into them. I run 1 3/4" headers 4-1's. As my plan is to have 408ci under my CHI's
For the moment I'll play with the 351.