My first instinct was that this would seriously break things once you got past the 3025 era, but having done some messing around with the math it doesn't seem as bad as anticipated.
Under introtech, I guess the biggest concern is the ability to build a better Spider at 25 tons. Not a massive issue as it's only an extra 1.5 tons to play with, but I know I tend to avoid 5/8/5 heavies due to a similarly minor disadvantage so I thought it was worth bringing up. It also makes a 9/14/9 mech semi practical and a 10/15/10 possible, but it's certainly not the end of the world. That's about it for side effects on the game as a whole. (Obviously it makes the Wasp and Stinger better in relation to the Locust and a little less bad in comparison to the Firestarter, but that seems to be your intended goal)
Moving up to star league tech (Endo and XL Engine) it gives minor boost to 10/15/10 mechs when dropping down from 30 to 25 tons. It also makes the 11/17/11 and 12/18/12 horrible jumping bastards a whole lot more practical without having to involve IJJs or partial wings. 13/20/13 and 14/21/14 mechs are possible and a 13/20/13 Tag platform (or later C3 Slave) might even be useful.
3050 era Clan tech just makes the mechs nastier but doesn't really change the equation.
Partial Wings allow a slightly slower 25 ton mech to hit 11/17/13 or 12/18/14 with a little more tonnage than the 20 tonners 13/20/13 and 14/21/14. But they also allow the already allowed 9/14/11 30 tonner to enter the horrible jumping bastard category as with more tonnage than a 11/17/11 25 ton mech so balance wise this is very much edge case territory. At this low tonnage the difference between the clan and IS versions isn't significant. A 20 ton mech can hit jump 15, but if you want both TAG and any armor you need Clan tech. Jump 16 becomes just about possible and you can hit jump 17 with the aid of either mixed tech (Small Cockpit, XL Gyro) or Dark Age tech (Clan Interface Cockpit, no gyro) but I haven't found a way to hit jump 18.
If you've got IJJs instead of Partials Wings then you 30 ton 7/11/11 mech only has a half ton advantage over the 25 ton 11/17/11 with ultralights, but you can pull the same trick on a 50 ton mech and get all the tonnage you need. I can't find a way to actually use a hypothetical 0.5ton IJJ than doesn't either get beaten by a heavier mech or run out of crits. I was half expecting something to seriously break here, but
I don't think you can combine a PW and 0.5ton IJJs to do anything useful.
So in conclusion, I suppose the ultimate effect is to make a few mechs(such as Spiders) obsolete, to introduce some of the more ridiculous jump ranges of Jihad/Dark Age mechs into earlier eras and to make a few edge case high jump ranges possible. I don't think the game actually falls apart, especially if you don't allow customs, but I also wouldn't want to fight a 11/17/11 25 ton Clan mech during the invasion so if after modifying existing mechs you also allow new customs, I'd advise some caution.