@kevemaher , I have watched this conversation develop and I can see it might be of limited value to you. Feel free to call me to discuss. The first question I am going to ask you is what is your loading set at and what is the coil impedance of your cartridge. There may be at least three follow up questions from there.
For everyone else, I do not recommend changing SRA and VTA (Two VERY different things that are important for different reasons with different targeted ideal ranges) by increasing mat thickness as it changes a minimum of four variables that can impact the playback performance: SRA, VTA, tonearm pivot vector forces and record to support surface mechanical impedance. (Note that I am assuming you will adjust for VTF when changing arm height or record height otherwise you will also be changing damper compression vertically AND horizontally in addition to any changes to SRA & VTA due to the new VTF.) On some tonearms, height changes can also affect azimuth. If you make height changes at the tonearm pivot area instead of at the record, then you reduce your variables by one. Not a huge win, but a step in the right direction.
When attempting to make an improvement to performance, it is important to impact as few variables as possible so you can better understand and control the outcome. Without such control, you could - for example - change record height with a thicker mat and three parameters might improve but one might get much worse, giving you no obvious net improvements. Rather, if you were to do your best to limit the total variables impacted when you make such a change then you can assess for the changes in a more controlled fashion.
With that said, I think the likelihood of your issue being a mechanical one is no foregone conclusion. We can discuss when you call me. Phone on website. I’m not going to push my tools on you. I’m going to help you control the variables so you can get to the bottom of this. High frequency "screechinesssssss" is NOT a characteristic of moving coils that are properly setup and electro-mechanically damped.