12 core 2.7ghz vs 10 core 3.0ghz

clock speed is not an absolute measurement, and it's not uncommon to see clockspeeds drop as cores go up

if you are going to be working in PS it's mostly about CPU and only a few things are optimized for the GPU at this time, and Apple has gone out of their way to make Nvidia a nightmare to work with in Mojave so AMD is your out of the box option but it does seem more and more GPU's are becoming the driving force behind Print and Video and 3D

https://www.pugetsystems.com/labs/articles/Photoshop-CC-2018-NVIDIA-GeForce-vs-AMD-Radeon-Vega-1197/

wait, are you really out of processor power?

have you looked at activity Monitor CPU pane when you are doing your most intense work, and seen that there are no more CPU cycles to be had? I am skeptical.

Many Mac Pro do not have enough RAM to be as productive as possible, and many do not have multiple drives to accommodate a smooth workflow -- they are waiting for I/O, not processor speed.

Be certain your have optimized those cheaper things before you blow a wad of money on more processing power.

Some apps are still designed or by nature only suited to using one main processor thread and hence do not benefit from extra CPU cores. An example is the infamous Retrospect backup software. So for these types of app your right to look for the faster fewer CPU cores.

However modern photo and video editing software is more likely written to benefit from additional CPU cores. These types of apps also benefit even more by having newer and better video cards.

Unfortunately the video cards in the Mac Pro 2013 aka MacPro6,1 cannot be upgraded. You could in theory look at an 'eGPU' that is a video card in an external box connected via a Thunderbolt cable. Even more unfortunately eGPU solutions for the Mac do not properly support Nvidia video cards which are the type Adobe PhotoShop and Adobe Premier and Adobe After Effects prefer.

If your using Apple Final Cut Pro then this prefers AMD video cards and hence an eGPU using an AMD Vega card would potentially help.

I would first echo the suggestion of Grant and advise you have Apple's Activity Viewer running whilst using your Apps and see if all or most cores are in heavy use or just one or two. If only one or two cores are maxed out then the problem is single core speed, if all cores are maxed out then the problem is number of cores.

By the way the definitive list of CPU upgrades for the Mac Pro is here - https://forums.macrumors.com/threads/mac-pro-cpu-compatibility-list.1954766/