Hi Bob,
I assume you want to keep the same prop to maintain the same lap speed which begs the question why did you change the engine in the first place.  Anyway I have a couple of Windy's videos and he goes into this very subject on one of them.  According to Windy the best reason for more engine is it gives you more leeway in adjustment for the engine.  It make the engine more user friendly and less a chore to dial in.  He also says that larger engines are working less to achieve what the smaller engines are giving you and since they are working less there is less chance of over heating and running away. 

Also there is a power surplus that when condition require can be tapped into.   Things like windy days.  (no pun intended)  I found this very usefull this past weekend at a contest.  I was flying P-40 and was the last flight of the day.  The wind was staring to blow and I reached for some additional power to get through the pattern.  If I had already been maxed out I would not have flown the flight.

Anyway that what I understand about moving up in power