[Hectic]

Ya with a 9.6 (not 9.4) you will notice lil better trigger response and rate of fire and with an m16 you have a tonne of space so you can use whatever battery you like most any one will fit.
And a good hop up rubber will make a difference in you range and accuracy most stock rubbers are prety crappy. Try a flat hop setup (a rubber with no nub and a flat or bridge bucking) you can get both for under 40 bucks with shipping.
Buy a good battery and charger.
Id wait for a metal body till yours cracks. Just an example i have a cyma mp5k with metal body and an echo1 m4 with a plastkc body and 12 inch rail and my mp5k (a short ass gun) is way heavier then my m4 (about the same size as your m16) so if you dont mind lugging the extra weight go for a metal body otherwise just stick with plastic.
That or you can just buy another gun (lord knows you can never have too many lol)
Maybe the AKM you wer lookin at (the lct akm full metal and wood 400fps out of the box stock 6.02 tightbore i think mayne 6.03 is around 400 bucks or so.
Cyma is cheaper they are still prety nice tho lil lower quality on the wood and metal and internals
Or a small gun like an mp5k or regular sized mp5. (again if you go cyma they are not too bad not great but not bad also metal where they are spose to be and in the 2-300 range.
There are a tonne of options.
Bottom line dont fix what aint broke.
If your gun works good use it if sokething breaks fix it
Tightbore (a good one that has a consistant bore like a pdi or prometheus or madbull.
And a good hop up rubber like a prometheus madbull lonex even a guarder
Those two things will make your shots more consistant and increase your range and accuracey you can do both for under 100 bucks depending what barrel you pic.
A promy bearrel will prolly run you 60-80 bucks and thatll prolly be the most expensive of the bunch