I've had three modded engines in my nitro rc career
1st was an ABmodded OS .18tz I didn't personally run it but the person who bought it said it ripped and ran cool(don't know how they perform stock)
2nd was a Powerhouse modded Picco .28 this motor was absurd with power and ran cool to boot..I had to go up 2 teeth just to keep from topping out early and to have a smooth feel great gains over a stocker.(sold it on ebay way to much power for the track felt like more of a drag motor)
3rd A PTM Ofna Picco 3 port(RTR engine actually) was pretty fast in stock trim and I decided to go ahead and get it modded...once again vast improvement top to bottom went from being a 6 minute motor to a 7 1/2 minute motor smoother delivery and a noticeable top end gain(was it worth it yes I think so)
I am no expert by any means but I do have enough knowledge to tell the differences in the same truck,same fuel,same setup etc.
As many have stated in the higher end engines you probably won't notice much power difference but the fuel eco my be better...with that said would I mod a speed NO a blue head vspec,plus 4 or something along those lines yea