Interesting timing on this thread as I am asking myself the same questions....My port drive grenaded 20hrs after a rebuild, took the case with it too. I have 500EFI's making about 530hp each. Now I'm wondering if I should goto XR's...

As Curtis says.. driver experience is the cheapest outdrive upgrade you can make.
I run a 28' Checkmate that is some where around 6k lbs. Friend of mine has the same boat. Similar setups. His is 598" mine is 565", both have stand off boxes. I am running a B1 with XR upgrade. He is running a stock B1 1997. He ran 3yrs with 330hp and then 4yrs with big power. I tore the drive down and all the bearing were shot. Gears were in good shape. I rebuilt it in 03. Still running strong! I attribute that to good driving habits.
A properly setup B1 will go along way. X gears and steel towers and good shafts in a useable B1 case will hold up to a lot, if driven with respect. I have built several that way.
I like to be more aggressive when I drive, so I went with XR gears and they have held up well. I dont have as much time on my drive, approaching 300hrs. Not sure how much power each of us is running.. My friend went 91mph at Loto. I have seen 87.7mph, my best so far. And that was with 5 bad cam lobes. So anything is doable!!
My theory is that if you dont have enough power to knock the teeth off the X gears then they will last longer. More power, go to the XR gears, but be willing to replace them as needed per your driving habits!!

Hope this helps.
Dick

Left hand rotation drives (Your port drive?) are at a huge disadvantage. The "top" gear is taking all of the torque and is only supported by the topcap. (Unlike the lower gear that is actualy supported by the case tower. To make things worse, the top cap bearings and races are "splash" lubricated. They are below the fluid line of the drive. This is the main reason we designed a billet, water cooled top cap. Better support, stroger tower (Billet material with a huge radius at the base) and much cooler oil temperatures. A must for L/hand rotation drives. Not too bad on the rights either.
Curtis @ Billet Marine

Do you have a website/contact information?