The batteries were 25C and lasted about 9 minutes. Just got warm, not hot. Tried up to 50% punch and it was still tough to control. Other weird problems were all torque related. You will have to grind a flat spot on the shaft. Even with red Loctite it will work its way loose. The tranny shaft that the slipper mounts on will snap if you nail it too hard. The outdrives will also twist after a while, keep plenty of spares on hand. Also, replace the plastic 44t spur inside the tranny with a hardened metal one. The plastic one smokes almost instantly. The diffs hold up fine if you use the new hardened ring and pinion set from HPI. Let's see, what else. Oh yes, the only wheels that don't deform into an undrivable mess are the Pro-line MTR's with the molded foams. Any other tire and strapping tape is needed just to make it drive straight.
Honestly, the 1521 is better. More power than you can use without all the problems. The 1527 will have you repairing something every run, and I mean EVERY run. The trucks just aren't made to handle the power. I figure it is like the dual nitro engine setups, more trouble than they are worth. Anyway good luck you will need it!