To get right to the point, fletcher is overpowered.
The reasons for this are he has access to some of the best weapons in the game, both primary and secondary, has sticky bombs which ohk (almost) every class, and where if you have the aim above that of a toddler will be as effective if not more than fraggers and naders, can clear C4 points with one or two bombs that, again, OHK every engineer class and almost oevery other class,and on top of that can plant and disarm c4 twice as fast (give or take) as most other mercs.
I think that the best way to balance him would be to, instead of nerfing his ability to kill, would be to make it so he cannot have as much if not more killing potential than naders/fraggers/thunders, or even rhinos if its a decent fletcher while also being able to do objectives at the rate of the weaker engineering classes.
Give him an ammo station instead of ammo packs and maybe buff his HP to 120 also to fall in line with the other ammo givers (save kira)
Id even go so far to say that rhino is more balanced, because yes head on he has more killing potential but he cannot do objective at all, pretty much needs a pocket medic unless the other team is bad, and fletcher can get kills out of LOS easily