The Diavel will be more flickable imo and have higher potential lean angles before touching down.
My .02 new vs old geometry:
The new rockets still has silly geometry with a rearward rake (reverse rake) where the rear axle is lower than the front.
Shortening the trail, making the steering angle sharper and slightly shorter wheelbase will certainly make it corner better than the old model (when stock). I imagine the journalists are going to type the following ad nauseum: "a bike this heavy has no right to handle this well" and "somehow by some miracle Triumph have made it's 294kg feel like 194kg once moving". It will undoubtedly be fun to ride.
When you consider the geometry of a RIIIR/Classic/Standard after putting on a 240/55R16 and 140/75R17, plus 1" longer shocks, the geometry numbers are almost identical between the modified RIIIR and the R3R/GT (I know, I've measured mine with such a setup).
It does handle quite well once setup in this manner, but the 240 is always going to be a limiting factor in cornering radius and the weight is always going to hurt flickable perception, that will still be relevant with the new bike.