Lev
It's my personal opinion only but I would suggest that leverage should play NO part when managing a personal pension.
Use leverage sure but only with money that is speculative not money that will be used to pay yourself on retirement. With a SIPP you don't have to make big money, earn 10%-15% a year over 2+ years compounded and the pension pot will really grow.
I'd suggest that over 80%+ of those who use leverage in a SIPP will really regret it at some stage over the next 10-30 years (about the time of most SIPPs). And remember, you only have to be unlucky once to pay the price.....