500, the weight is there to lug about, but with smokeless powders a full velocity 8-bore load in a 16-17lb rifle is no more "painful" than a light 458 in terms of feel, to my subjective opinion. I've honestly shot poorly stocked 300wins that hurt far far more on the bench than an 8 off of sticks, and there's no comparison when it comes to style. If you can shoot a light 338 or normal 416 (and I know you do and more), I think the weight and lower acceleration rate make it 'feel' comperable despite the higher theoretical recoil ft-lb numbers. Off sticks 2 ounce balls at 1600 and 3 ounce concials at 1325 are reasonable fun loads with smokeless. Try it, you'll like it, trust me!
Bob