When beating Kauto Star in the Gold Cup Denman recorded an RPR of 184.
When finishing runner-up in the Gold Cup he recorded 173 and 176 (but in the first of these he had been a sick horse all season).
His other three best performances in non-handicaps were: 178, 169 and 165 at Newbury and Cheltenham. In the Aon he beat Regal Heights 20 lengths and Ollie Magern 80 lengths, so you could make up any number you like for that.
In his three Henessys Denman recorded RPRs of: 183, 184 and 182. Imperial Commander, Kauto and Long Run will have to pull out all the stops to replicate this sort of mark, that I'm confident Denman will run to.
Denman has therefore recorded four ratings above 180, but as you say only one in a non handicap. However you make too light of his Hennessy performances, these are the best handicap performances since Arkle.
He suffered from going wrong at a crucial point in his career. But is now back to something like his best, which is why I think he’ll still take the beating in the Gold Cup this time even if not at his absolute best.
I believe his current Timeform rating is 181 and his OR 182. Imperial Commander, Kauto Star and Long Run will have to pull out all of the stops to replicate this and I'm confident Denman will be able to run to between his official mark and 185 in the Gold Cup this time.