60k is the rpm limit, if you spin any faster your warranty and reliability are out the window, heat becomes an issue, and it could become a game of keeping replacement parts on hand :-)
This is also why I recomend the XL over the Mamba Monster in your rig, reliability as it has a higher amp capacity and data logging so you KNOW what's happening after a run.
If you don't care about ratings and reliability, you could run a 1415/1D (4200kv), I've heard they will spin 100K for a while. Pair that up with a Mamba Monster on 6S for 94k rpm, gear for 100mph...How long it will last, and how hot it will get are a mystery to me.
I know the 1518/XL combo I have is rock solid, runs cool, and is IMHO the ultimate setup for a <10 pound 1/8th buggy.
P.s. There is a way to make the 1518 fit in your buggy
You could run the XL on 6S with the 2200kv Castle 1515 too