Four Wheel Stampeders has had a 40" tire limit for a couple years now. There are a few reasons for it. 40" is kind of a magic number where once you reach that point your rig is no longer "trail rig" but more of a hardcore machine(big axles, HP to turn it.....) more suited to competion.
We have found that all the trails in Waiporous/Mclean/Dutch Creek/Indian Graves that we have done can be handled with a 35" tire. 90% can be negotiated with 31s and a winch. I know I've done it. You just have to be ok with the fact that you cann't make it thru every hole or even turn turn back. To build a rig that can simply leads to bigger holes and trail blazing of new trails. You get bored of the regular trails and start looking for new stuff.
The simplist way of dealing with it was to implement a tire restriction. 40" was picked as that seems to a natural break when you go from simply modified to wild. As well a few members were currently running 39's so this didn't exclude anyone.
This also keeps all over our members on the same level. Clubs in the past have fractured due to the "big" guys getting fed up with having to wait for the "newbies". Some clubs in the past have even considered having a "minimum" tire restriction so I really don't think that a max tire size is all that crazy.
It also shows SRD that we are trying to take measures to reduce our impact on the trails. Like it or not the bigger your rig is the more SRD frowns .
As to HP on a trail rig... HP has absoultly ZERO to do with how capable a trail rig is. Proper gearing will take you much farther on an obstacle than big HP. I did all the big name trails in Moab last year with my stock 4.0L, 5.38s gears and 36" tires and never felt the need for more HP. Neither did the two Toyota pickups with 4 bangers and 38s. Last year at High Riders Jenssen placed second with an XJ and a 4.0L. The only time I've felt the need for more HP is running on the highway.
Big HP with 40+ tires say only one thing to me.... mud boggin, sorry but those days are gone.....