I must say my experiences of this matter are similar to tazarooni's. I have experience of two RG unis (Cambridge and UCL) in the same subject. At Cam, it was a constant uphill struggle, not only was the coursework, the exams and the content far more taxing, the material went in to a lot more depth as well despite covering the same topics on the surface. This combined with the fact the terms were chronically short/work time was compressed meant I found it a massive struggle to just get that 2.1. Instead of doing my 4th (MSci) year of my degree at Cam, I switched to UCL to do an MSc instead in the same subject, where I was doing the same as their MSci year plus an additional module, a lit review and a longer project (it extended into summer). Here I easily cruised to a high first/distinction (86). In fact I was taken aback in the drop in difficulty.
Despite having only experience of these two unis, this and other evidence/listening to academics has lead me to believe that yes, there are vast differences in the difficulty of some courses at different institutions, and if you carefully select your university you can get a higher grade than going to another.