Quote - I saw a massive difference when I moved from dual to Quad core with H/T,  both in rendering times and the size of scenes I could build without any problems.

The problem is how you do the comparison, you are not comparing two with four cores. When you moved to quad cores, you had a new processor with a new processor design, a new motherboard with a new chip set, faster memory with greater memeory bus speed and more memory and moving from 32 to 64 bit software also can be significative.

Every time you buy a new computer or processor + motherboard it will run faster than the previous older one. If you buy today a new computer you will not be able to find a single one that has less than four cores, all are multicore today.

If in 2013 fabricants decide to redesign the processor and return to only one core, it also will run faster. This will not mean that a single core run faster that eight cores, it only will mean that a 2013 processor runs faster than a 2011 processor

Stupidity also evolves!