After whatching your video I think your only problem is that you are afraid of letting the engine scream at high rpm If you want thrust at LOW rpm then you really need a fourstroke and a bigger prop

This is my recipy to get monsterpower from my XLS 52:
Remove the baffle inside the muffler, or even better order the Tower 46 muffler and put it on your .52 engine for serious increase in power.
The XLS actually love nitro and the more you give it, the more power it provides. I fly 30% helimix with great success, but it is still very powerful on 15% too.
Get rid of that master Airscrew prop and try both the APC 12-4 and 12.25-3.75.

Still not satisfied?? GET A BIGGER ENGINE