My understanding is more blades is more prop surface area in the water. Less prop slip.

Finally you take an answer from someone ? I have the same question.

At that means more pain for the drive. If the torque is not slipped away, something between propeller blade and connecting rod will be the weakest link.

I would think that the XZ gears would be a help. Less expensive to upgrade and not have a failure than to have the internals polluted with metal.
Have you considered doubling up on drive coolers? This aims a pretty good stream of water a top cap and to a degree, a lot will go down the sides. More is always better!
Pretty reasonable price, my son put one on his B3. Observing it at speed there is a chit ton of water splashing around back there.
https://www.ebay.com/itm/28221628448...saAug3EALw_wcB

I've never seen anything quite like this before. pretty cool.

It does the same thing functionally as this, possibly not as effectively but for the price. On my Son's B3 he will be using the Halo and this. B3's get hot!
https://imcomarine.com/cal_store/bravo-shower-cap.html

I have not. Still waiting to get my engine first. Looking like I will have all winter to figure this one out unfortunately...

Excatly. More prop in the water has less slip.

Less slip means more stress on the drive.

More stress on the drive means better chance to break drive components.

OK, lets try this.

Rebuilt XR or brand new XZ?

4500 lb boat.

502 cu in with 600 HP/ 600 TQ

Recreational use with occasional WOT runs.

I would go with an XZ or even just an X. XR gears may be slightly stronger, but probably have half the longevity of X gears.
New XZ's are not sold anymore. You would have to buy a Bravo X upper and XR lower to make an XZ.