(should go in House Rules)
I don't think it's a good idea to use ability requirements as a balancing factor. The Bastard Sword and Dwarven Waraxe don't require 13 STR anymore to use. Also, you fail to mention if it is a Martial or Exotic Weapon.
If you compare your weapon to other bludgeoning Two-handed weapons out there...
The greatclub weighs 8 lbs and does 1d10 (20/X2), but is probably the "worst" 2-handed weapon.
The Heavy Flail weighs 10 lbs and does 1d10 (19-20/X2), gives a bonus to disarm checks, and lets you make trip attacks. Better.
The Maul (CW, Martial 2-handed, Exotic 1-handed) weighs 20 lbs and does 1d10 (20/X3).
The Warmace (CW, Martial 2-handed, Exotic 1-handed) weighs 10 lbs and does 1d10 (20/X2). You suffer a -1 AC penalty when wielding a Warmace.
The Goliath Greathammer (RoS, Exotic 2-handed) weighs 30 lbs and does 1d12 (20/X4). It gives a +2 bonus to sunder attempts.
I think 2d8 is too much for a medium 2-handed weapon, even if the crit range/multiplier is only 20/X2. I'd let 2d6 slide, but not 2d8.
AR