A couple of the Ponsse guys have said that in the taller bigger diameter stuff the biggest harvester (bear) had all it could do to move the trees. I think the scorpions and ergos are doing smaller growth and early tsi thinnings. I honestly believe a scorpion or ergo could do everything your 653 can, and do it faster.
Now, the technique would be considerably different switching from a fixed to a dangle. Some guys try and just cant do it. Or more likely are so stuck in their ways they refuse. If you are of the mind that a dtangle head just won't work thats a pretty good indicator that you fit in that group. There are proven fixed heads that are in the same league as far as speed, like the logmax 7000 fixed.
If you need a head that will work wood in the 30 inch range all day, the wheeled machines won't do it. Thats Waratah, southstar, keto etc on a tacked carrier territory. And volume I can only dream about.