I personally splashed out for the 8 core Intel processor on the basis that I use a lot of effects + VSTi’s and these would benefit from extra cores rather than clock speed. But I have been watching the various discussions about how well cubase uses cores and I have to say that I’m not convinced either way, and I’ve not got any evidence either way from my own system so I can’t really comment! My other reason for splashing out is that I wanted to overclock and so I’m overclocking to 4.5MHz, which hopefully gives my the benefit of fast processor as well as 8 cores - so I cover all bases 
The processor alone cost £850 (Core i7 5960X), but I’m fairly sure it will still be able to deliver enough power in 3 years, so the cost is worthwhile in my opinion. Before that I had a 6 core for a similar price, similar reason, and I was starting to max it out. However, the new system’s power is not just to do with the CPU clock or the number of cores because from what I’ve observed it would seem that the latest memory and the latest chipsets/mobos really provide more performance than just the CPU can, so there’s a lot more going on under the hood I’d say. In other words my new PC is way more powerful than the clock speed or the extra cores would imply… My general philosophy is to pay up front for something powerful that will give me headroom for next few years, so I purchase the best I can and it has served me well over the years.
Mike.