When you still have the same ashmatic head, extra capacity doesnt result in anywhere near a like for like gain, so a 25%+ increase in capacity wont yield anything like that sort of increase in bhp, but it will allow you to consume the same amount of air safely at a higher cr and hence drive nicer off boost, so although the peak bhp might not increase much, the area under the torque curve if you look at the whole range will increase with the extra capacity.