+25% damage means little considering RE have horrible accuracy, even with veterancy unless the target is in front of you.
Then why implement it at all? The only units it really affects are the kubelwagen, 222s, and increases the RNG effect due to their said horrible accuracy. It's your mod, do what you will with it, but I think vipper does have a point with the kubel, especially if the kubel is OKW's starting unit. (I'm guessing this is a direct result of that?)
Rear Echelon can't lay mines, their flamethrowers (now that they get them) are doctrinal, and all USF vehicles except for the Calliope and Pershing have vehicle crews to repair. They can't even lay sandbags. Though they can place tank traps, those aren't necessarily the best thing to litter the map with as an army of medium tanks.
These are the reasons Rear Echelon aren't the most viable unit. Wouldn't it be worth exploring addressing those issues?
Have you considered giving Rear Echelon access to the Assault Engineer mines? Have you considered locking it behind the minesweeper upgrade? You could even make the minesweeper a sort of demolitions package that gives them a form of demo, or even that british cover destroyer despite my loathing of that ability's concept. Mines are key, and RE's inability to lay them always felt curiously crippling.
They'd be good candidates for an upgrade that increases repair/build speed and grants them the repair vehicle critical, and then scrub it from vehicle crews altogether. They could even make sandbags. Mutually exclusive from minesweepers/mines, of course. It'd give them that unique viability that you might want an RE squad nearby for.
In my opinion if the if RE carbines are to be combat viable instead of all this, they should drop down to a 3 man squad. Give them flamethrowers stock and change doctrinal flamethrowers to one of the passive rifle upgrades. Either vet 2 or 3 would be appropriate for the 4th man.