If your workload is already well parallelized, then, yes 20% is quite significant. However, working to parallelize properly over 8 rather than 4 has its own costs.
The thing that bothers me most is that 800% CPU and 500% CPU on this processor are roughly equivalent at 5x100%CPU, it makes everything very hard to reason about when planning capacity.
If I had a nickel for every time I had to explain why "You are at 50% CPU now, but you can't actually run twice as many processes on this machine and get the same runtime", I'd be able to buy a large frapuccino or two at starbucks.
Perhaps I'm uninformed though - is there a tool like htop, which would give me an idea of how close am I to maxing out a CPU?
As far as silicon/power it is nice, but IIRC (I am not involved in purchasing anymore) it used to cost over 50% in USD for those 20% in performance when you non-HT parts were common.
It (used to be) my job. Does "because people fall for deceptive marketing, waste money, and then waste my time trying to salvage their reputation" sound better?