anybody swapped them out and left everything else equal???

We did a swap from a velvet 71 c modified with more clutches to a crash box ,it was on a single engine 28 ft boat with a 750 HP BBC blower engine ,we could not see any difference at all ,no rpm /speed gain nothing

What was the weight difference?

I think its largely reliability that you gain from the crash boxes. Pretty much indestructible.

On the Huber side would be dockside manners, on the crashbox side (as GlassDave pointed out) durability. I would think powerboat speeds would remain equal between the two.

I have no evidence to back this up but i have been told a normal transmission robs 100hp more from a motor when compared to a crashbox (in this example behind a 950hp blower motor). no idea how that translates to differences in speed

I can't see that being the case. To the best of my knowledge a trans usually only eats about 35Hp. Boat transmissions are pretty basic, especially compared to automotive transmissions with multiple gears. You really think a Corvette or Mustang stick has an extra 100hp over an automatic?

Everything I've seen and know suggest that it's only about a 25-35 hp loss to run a transmission. A crashbox is going to eat something, but given that the total loss is 25-35 the difference in Hp loss is going to be minimal.

We did not weigh both to see the difference ,but out of my head I think the crashbox is 20-25 lbs less

If it has a cooler then there is a power loss,via friction generated heat.

Steve 1 - very astute remark

If it makes enough heat to need the oil cooled it's taken some Hp to make that heat - the more cooling the more Hp - good analogy!