After a few races in the VJ Xtrm I've yet to notice any major difference between the grip in these and the superb grip of the Irocks. i.e the grip is fantastic.

The comfort however is vastly improved due to the inclusion of the rock plate.

In short, these appear to be an improvement over the Irock, and are hands down the best fell shoes I've ever worn.

2 points to consider before i get too carried away...

I've only had them since xmas.
and
They are still a minimal fell shoe. However after running some long hard races in the Irock, these are easier on the foot.