Last year a bunch of craft, led by a Griffon 2000, did a circuit of Portsmouth harbour at 10 knots. We were under strict instructions not to pass the Griffon which kept the 10 knots - and no-one did. It's do-able.
Technically, the hump speed depends on the craft length - it appears at a Froude number of 0.57, and works out at 6 knts for a 10 foot craft, or 8 knts for a 18ft craft. Having said that, this is theory and not many hovercraft have read the books!
The key to it is being lightly loaded - with a low cushion pressure the hump wave is less steep and spray is less.
Ian