If your interested in a car with 300+hp, you bought the wrong car. I don't think anyone has added 100hp to a Boxster (but I may be wrong), but if it has been done it cost well over $14k to do. Boxster's weren't designed for drag racing, they were designed for high speed handling. Looking at the cost to benefit ratio you might be better off, with another car if you want 100hp more.
A 3.4L is possible, but it won't deliver the hp you want.
Just my $0.02!
