What distances are you looking at? 6.8 SPCII is a lot of gun in a light package. Any
20" 7mm-08 would be much heavier for maybe 400y gun vs 300y 6.8 (medium game range).
If you're giving the benefit of the doubt to the 6.8 at 300, then a 7mm-08 is easily 500-550, not 400. Federal 7mm-08 with the 140gr Nosler BT has more energy at 500 than the Federal 6.8 MSR 115 gr does at 200. Granted that's from a bit longer barrel than 20" but it won't make that much difference. The 7-08 packs a serious punch. I have both and the difference is very noticeable.