On a JG G36 ( cause it was my first and been my primary for a long while now)... the internals aren't set up for Lipo. You'd get about a 9.6 out-put from a standard battery , maybe a touch more than that , using a 7.2 Lipo , it wouldn't exactly tear up the internals any more than a 9.6 would. But it will cause the internals to operate on a higher current out-put, and will break down faster than using a normal battery.
In order to "lipo ready" the JG, it takes some money, and some know-how to DIY.
First off, JG/Echo-1... not that great on getting grease jobs very good. They usually either have too much, or not enough. To run a lipo , you really need to have the almost perfect amount of grease for the gears to operate smoothly.
You also need a motor that the bushings aren't going to burn out in, and the pinion strip or break on the gears. It's questionable on stock JG/Echo-1 motors if they are quality enough to run a Lipo for very long. Thats hit and miss, as JG tends to use what motors they can get cheaper... some are better than others.
I forget if you need to replace the gears or not too....
Anyhoo... The gear box needs to be REINFORCED. what happens is... higher ROF, and harder hitting piston, keeps slamming into a gear box, and can crack the front end of the mech box if it isn't tough enough to take abuse. Again, thats hit or miss at the quality of mechbox JG used. ( Unless you know it's already re-enforced 100% with out a doubt. )
You gotta re-shim the thing too. So nothing jostles out of place. It helps to have high speed 7 Mm or 8mm bearings as well.
I might be forgetting something else, but I know thats the minimum you gotta do to lipo ready the gun. Other wise, what would last you a few years, will probably only last you six months or less, depending on how much it's used.
Now that said... 12-15 RPM is actually wonderful blessing. On top of what was already said... you can actually count the shots when auto-firing , and use controlled 3 shot burst easily.
With-out having to put a mofset in to do it for you. You start getting 20-25 RPM, the BBs fly out so fast, you can't keep track of your ammo counts accurately.
If ya want to go into a role where you are support and suppress... invest in an M60 , or Saw and those sling large amounts of BBs down range, with decent ROF. Or save up and get say one of those c mags, attach it to a AEG with a higher ROF already, Like an M16 , or AR 15 model.
Getting a lipo, and converting a clone to accept it, will run you about as much in cost ( parts and labor if not self done) as it would to buy a lipo ready AEG. A Stock G&G, ICS, or KWA are pretty much Lipo ready stock ... maybe have to do something to the mofset and up the fuse size, but higher end AEGs handle more power alot better than mid level clones do.
Not to mention that on top of gun upgrades, you NEED a SMART BALANCE Charger for any Lipo! Not an option to skimp in that area... the battery will explode, catch fire, or melt-down if not charged properly. Will. No Maybe. It will. Those smart balancers that are any good, aren't exactly cheap. Buy a cheap one, and you run the risk of exploding batteries almost as certainly as if you hooked it to straight 110 volts directly to it. ( I might be exaggerating a tad, but just to get the seriousness across of what I mean. )
Handled right, and careful watched.. and balanced. Lipos ( especially 11.4s) Give any AEG a boost in performance. They do last longer, aren't effected by cold... but you got to take very, very , very good care of them.
A better solution to a Lipo for your G36 to increase ROF... is to buy an after-market high torque motor and drop it in. I bet Jon would do it for ya ( or his guys) for a small amount, plus cost of the motor.. and most likely fix it later on if something fudged up. It'd up your stock ROF a bit that way, and you could still use 8.4 and 9.6 volt NiHm batteries.
( On a side not, a new better hop-up and bucking and tight bore barrel will increase your accuracy, and a new piston head and cylinder .. possibly a spring guide too... would greatly increase accuracy no matter your ROF.) More accuracy, means more hits in less BBs spent.
Hope that helps a bit.