Depends. Best thing you could do is get a proper speed fin if you don't already. Something around 25-26cm. That should add a few knots on in ideal downwind conditions - but make sure you're sailing very broad and overpowered. Ultimately, you need to work out if a speed board will be used enough to justify the expense. I think the answer you're looking for is
yes, a speed board will always be faster than a slalom board in good-ideal speed sailing conditions because of the reduced water and wind drag, flatter trim and optimised outline and rail profile.
The other question you need to ask yourself is "Am I having enough fun sailing at 36knots on my slalom board?" or do you want to aim for the best speed you can achieve in most conditions and sacrifice other things like upwind performance, safety when the wind drops out, ease of gybing, performance in chop etc.
I have both. Covers all scenarios. Hope this helps.