It takes more than just cores and clock speed to have good VM performance. If it were entirely down to clock speed and cores, the performance would have plateaued long time ago when the base clock rate essentially didn't go above 4GHz and core count pretty much stayed at 4 for consumer desktop/mobile CPUs.
If you compare for example a Coffee Lake i7-8700 and i7-3770, you can hardly see the things that will make a difference from the Intel ARK site. In fact the base clock speed is slower.
https://ark.intel.com/compare/126686,65719
The core count and amount of RAM has a greater effect on how many VMs you can run simultaneously, comfortably without the fans running at full speed. 16GB RAM might be even sufficient even if two VMs are powered up simultaneously with 8GB allocated to it. There is a point of diminishing returns with RAM (both physical and virtual).
It is hard to give advice on a specific CPU to use and it is hard to predict the future as to how CPU technology will advance or regress. Part of the problem is Intel does not publish specifications on certain things (maybe they do and I am not looking at the right places or maybe it is behind an Intel registered site). One of the things that Intel has improved on is the amount of cycles it requires for VM transitions. So a Coffee Lake CPU will likely require less CPU cycles for a VM transition than an Ivy Bridge CPU. The frequency/need fo such VM transitions have also been reduced. It is these types of improvements that can make a 2.5GHz Haswell CPU have VMs likely outperform than if it were running on 3.6 GHz Westmere Xeon (just for the sake of example, I didn't make any actual benchmark).
One warning about Kaby Lake/Coffee Lake CPUs, you may need to upgrade to Workstation Pro 14.x if you are still on an earlier version Workstation software. If your development VMs require enabling virtualised performance counters, VMs will not be able to power up under Workstation 12.x or earlier with Kaby Lake/Coffee Lake CPUs. I cannot confirm whether Coffee Lake CPU is already supported in 14.1.2 but I see the Coffee Lake string is now the vmware-vmx.exe of version 14.1.2.