What sort of mods? Speed mods? If the airplane in question is already slippery, probably not by much. Highly incremental for the most part.
I've been in the Comanche world (mostly Twin Comanche) for most of my "GA life." I've seen, heard the stories of, flown and compared them all. The airplane can be made a little faster with a couple of the Lopresti/K2U mods. Debatable whether they're worth the cost. Sometimes there are other benefits besides speed. I like the TwinCo "WOW" cowls. Much easier to remove when you need to work on the engine, and they don't disintegrate like the stock nose bowls slowly do over time. They are theoretically worth a knot or two of true airspeed. 5+ knots - no way.
As is so often the case in life, the "common sense" mods win easily. For the airplanes with dual fork landing gear, swapping to single fork reduces drag and gives back a few free knots. A permanent speed/fuel efficiency mod upgrade which will usually pay off.
After that, getting the airplane in rig helps. A little off rig, you're slow. Mine was for a long time (still is, sometimes, I think.)
Keeping the airplane shiny and clean is worth a knot or two.
By far the most impressive speed mod for the TwinCo is adding tip tanks to airplanes which don't have them. There aren't any speed mods out there which come close to eliminating a fuel stop on a 7 hour flight. That's good for a 30-45 minute reduction in total trip time, if not more.
As for the rest of it, I haven't seen a whole lot of value for the cost. If you want a faster airplane, buy a faster airplane, all things considered.