The differences between high efficiency grid fleece and regular 100 wt fleece are noticeable and significant to me.
A simple test. Take a dropper, take a garment made with each of the above, turned inside out. Drop some water drops on each.
You will note that in the high efficiency grid fleece fabric, that the moisture rapidly spreads out over a large surface area.
In the 100 wt, regular fleece, not so much. This means the former will dry significantly faster even though both are primarily polyester.
Also, the former is now often treated with polygiene. Wear the polygiene high efficiency grid fleece on a 3 day trip as a baselayer, wear the the 100 wt on a 3 day trip as a baselayer, or even mid. Will be a big difference in stink.
Then, another difference, the high efficiency grid fleece will be both warmer with a windjacket, likely lighter weight, and even more breathable without a windjacket. All plus's in my book.
With that said, i think some companies far over charge for the really nice fleeces like the high efficiency grid fleeces--what Patagonia charges at full price for their Cap 4 hoody is borderline criminal---sustainable, good causes etc support or no. MEC is a lot more reasonable, but still expensive for U.S. and non CA residents because of shipping.
Btw, for me, it's not about going longer, faster, harder. Sounds like a porn promo in any case, and i'm not much a fan of that either. For me, it's simply about greater comfort and safety--in super cold weather, who doesn't like a super quick drying garment? (i actually tend to wear high efficiency grid fleece as a midlayer, so i don't get the flash dry cooling effect as much as when wearing it as a baselayer--i usually use a thin Dri Release Merino baselayer or the 55% Merino/45% nylon or 65% Merino/35% poly baselayers which dry a bit slower, and don't feel as cold when drying). I don't think there would be a world of difference between performance for us average folks, but could be enough of a comfort/safety differential to warrant some attention. With that said, decent quality 100 wt fleece is likely to be more durable and tough than the HEGF. Plus much cheaper, both also plus's in my book. But if you have the money, definitely go for the good quality HEGF's.