I think it all depends on what your expectations are. Sounds like you don't have a particular engine in mind, which is going to make it harder to answer your question. Some engines lend themselves well to modding. Also, a mod can be for a specific area of the powerband. Say if you have an engine with great top end, but a little underpowered on the low end, a mod can take care of that. Or, as noted a mod can make the usuable powerband wider overall, and increase engine efficiency.

Once you decide what you want, you can just ask whoever you want to do the mod if its possible.