DBCherry is correct. I would expect an increase in performance with a small amount of water. Water injection has long been a trick for high performance engines that are tuned towards high heat and detonation. Long explanation as to why, answers can be found on-line.
The glow RC engine is a high performance engine. The HP to Cubic Iinch ratio they produce is impessive indeed.
I suspect leaving the fuel in the tank is OK too. I do not do that for issues in transportation.
A big day of flying for me puts planes in all sorts of positions in the back seat and bed of my truck. I perfer not to have an fuel/oil stain.
Much easier for me to simply drain, run out the excess and clean before transporting. Cowl removal etc.

Debates both ways here concerning leaving the fuel in the engine. I don't and I remove the glow plug once home and put a couple of drops of my inexepensive pick of oil. Turn it over flip the prop to circulate the oil. Works for me.

Now what I do for batteries in my electrics...just kidding.

Cheers
Michael