I'm not sure if a 9.6v battery can be categorized as a modification. I run one in my SR-16 to help compensate a little for the heavier spring that's in it.
And, yep. LiPo batteries can do more with less space. I'm thinking about converting my M14 over to LiPo for better trigger response.
The trigger response is unbelievable for those of us who use your semi automatic selection more than your fully automatic selection. However, it has its other advantages as well. If you need to lay down some suppressive fire, the rate of fire it produces is often intimidation... and enough of it to keep the heads of your enemies down for however long it takes your team to move into position.
The biggest people most people have with LiPo's are the maintenance issues that go along with them. The primary being the charging stuff. If you let it discharge all the way... the battery is ruined. If you don't upkeep your gun... the gun will shred pistons, gears, and pinion gears quicker than you can replace them.
The reason behind this is the rate of fire being increased causes increased vibration on the mechbox which can cause your gears to possible fall in and out of place if not shimmed correctly.
So basically, if you take care of your guns... your lipo will take care of you.