Pitch on prop?

[Nasty]

If a 90 hp has a 14 x 18 pitch and pushes a boat 40mph, will adding 115hp with the same pitched prop add more top end speed? Or, should a new prop with 14 x 20 be added to get the full benefit from the bigger motor? I know a little about props... just quite not enough. Any help is appreciated! I won't be wet testing because it will be a new boat and I know someone with the same boat with that 90 and that 14 x 18 pitch. Thanks fellas.