The lead on the mainsheet down to the cabin top on the modified rig does set up a less than desirable line of strain versus running it under the boom forward to the mast. Given the purchase though, it's probably tolerable. Of more concern might be that the hauling part of that purchase will now be creating a spider's web of line on a reach or run, and a low obstruction to leeward.
One presumes it was done to prevent chafing against the attachment point of the rigid vang. I'd be inclined to build a bail or forked attachment point for the vang on the boom and return the main sheet reeving to original.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.