Are you soloing it or doing a team effort? Because of the gates, you'll not be going that fast anyway unless there's a team of you & you have a well-rehearsed 'gate plan'. Your mate is probably right that you will be marginally faster on a lighter bike, but not that much faster & you're better on a bike you're used to.
Two of us did it in Jul on 100mm travel HTs – injuries picked up in an early crash due to showing off slowed us down more than anything. A better question is what tyres for the 2' long setts paving the section between Hebden & the canal – those will be bloody slippery in the wet as they've been polished smooth by feet/hooves/tyres.
Andy