Yeah, already different sights and ironsights, without changing the ‘specs’, would be more than enough. It would simply be nice and add a little variety. The other things, as long as they change specs (too much), people will start to cry i guess
I do not know what they have against this, but imo if SC2 had not so many crybabies crying about OPness, the game would not be that successful 
Also the whole “you have more xp than me this system sucks because you are better now” is not something i find critical. I find it part of the fun, if done correct. It challenges you and it keeps you ingame, until you are on the same level or even higher than your opponenent. Of course if this system is not giving you the slightest chance to e.g. kill your opponent if he has a higher level than you, then the system is not working. It all depends on how you do it, there is no black and white, there are always different degrees. You should also not go to the extreme here and totally base everything on levels and make the game a xp grinder, where you spend 1000 of hours to get to the next level and to finally be equal to your opponent, no i do not think that would be favourable. I would rather see it as an addition, a nice aspect of the game which is not overstressed. It also depends on how you define an ‘upgrade’. There are upgrades which enhance your skill significantly, and there are others. In this case of weapon customization i would be careful to use the word upgrade. If you do it right, then it is no upgrade, but rather something different, with pros and cons depending on the situation.
My final suggestion is: look at BF3, look at PS2. Such a system is an integral part of their marketing strategy, to keep players motivated to play the game. It is kind of a part of the long-term model, if you will.
And my final, final suggestion is (sry) to introduce two game modes (two different mods) inside the game: comp mod and pub mod. This would solve the problem of having the game not played, because it is way too linear to keep the balance for the comp players.