It would actually be interesting to see, if there was actually any truly comparable data to show journey times / bunching likelihood for the XLBs vs any other single door decker. Sadly there isn't really though. Many of the frequent services in Edinburgh are always bunching simply because of the traffic and road conditions. Just one missed green light and having to wait can add 2-3 mins, and then with more people waiting, it just gets worse...
That said, I made a handful of journeys on the 11 and 16 and some of the journeys on XLBs did take longer. I think it was because, at the time, there was a bit of a difference in how they were driven. A few journeys, the drivers would open the centre door, let the passengers off, close the centre door, open the front door, board passengers, close the front door. And on all three of the 11s I caught where the drivers did that, another 11 ended up right behind us by Morningside.
The ones that seemed quicker (and on a couple of occasions, caught up with the previous service) were the ones where drivers would operate the doors London-style - opening both sets at the same time to minimise the time spent at the stop.
Although, based on today's punctuality mess, some services were upto and maybe over 30 minutes delayed. That surely can't be caused solely by a different vehicle operating the service.