Here is my take on underdrives.
Pro's
Less load on rear end
Cheaper Ring and Pinions (although when we went to get a set from Gear works the 4.11 was the same price as the 5.78)
More gear options (especially important with 40" tires)
Easier to quick change to Qualifying set up
Slower drive line speed, less vibration
Less load on trans
Con's
Expensive (figure at least $7K)
Need new drive lines that are shorter and have 1480 Universal on under drive side
Needs maintenance (fortin needs to be sent off every race to maintain warranty, its nice they have a warranty)
Moves Torque load to chassis and creates more chassis twist on acceleration
Still need to service 3rd member even though R&P are stronger the bearings and wear parts of 3rd still need service
More weight on truck- Underdrive, larger U joint, and mounts it adds up
HP loss in more friction and drag- I have heard from 10hp to 30hp loss with underdrive. But this is hearsay.
I am sure there are other things I am missing. I for sure don't know much about these. But this is a good starting point of a Pro and Con.
Personally I am glad we did it. I feel like our set up is strong and works great. Overall I think the change cost us about $15K all in with the Fortin. The fortin is great but my opinion is the Mason is stronger and a better unit but would have cost us over $20K all in.
Side note, We had a local shop modify our drivelines with the 1480 U and slip. That was a mistake as 2 failed at the Mint, that was the first driveline failure for us. We just got JEReel complete new drivelines made which is what we had before and are built right. don't mess with anything but the best with drivelines.
Mike