Margot---Good question, and the answer is, because the vacuum tube is self-leveling, you don't have to worry about any increased pressure. I've been adding the extra mat for dirty records on my HW-17F for years.
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One of the mods I did to my own VPI 16.5 was to replace the Mat. I also had a custom made acrylic platter made too, but that is not pertinent to your question here for the time being.
I replaced the cork mat with a 1/8" thick Neoprene Rubber Mat, of Shore Hardness of 60 I believe. Probably a Shore Hardness of 40-80 would be fine.
Supply houses such as McMaster Carr carry such. I first tried a self adhesive backed sheet, and had some issues with minor bubbling which I wasn't happy with, so instead acquired some sheeting with no adhesive, and permanently bonded the sheet to the platter with DAP Contact Cement.
McMaster Carr will sell 12"x12" sheets, and this is all you really need. First the Spindle Hole was punched, then smartly also punched the 3 Holes for Platter-Arbor Retaining Screws, thus won't ever be a need to remove the Mat from the Platter.
The black Neoprene is easy to clean, waterproof and chemical resistant, easy to see any dusts that might settle on the Mat, the Mat material is soft enough that there would never be any damage created to the record, and such will last a lifetime.
The Cork Mat on the other hand will eventually get dirty and old, and if like many, you're using some additional Mat for the flip side, this is extra work IMO.
My own machine can be seen at Osage Audio's site, under RCM Museum. Mine's the first machine on that page.
I also added a small AC pancake cooling fan as well to the rear of the machine. Yes, a little bit of work, but the machine runs absolutlely cool, no matter how many records are cleaned.
Spring strength shouldn't have any affect on the VPI 16.5, or other VPI RCMs that use the same Vacuum Tower Assembly with Helical Spring.
That any variance might more likely then be a alteration of Platter Height, and how the Vacuum Tower Assembly and Wand is adjusted.
That's one other main reason I chose not to use additional Platter Mats, due to slightly changing this adjustment-setting of the Vacuum Wand.
Harry Weisfield covered this once wither here, or in another forum perhaps. Here's basically what he said in a nutshell:
Place an average Record on the Platter and Clamp.
Lift off Vacuum Tower Assembly so that you can then remove the Helical Spring. Replace Vacuum Tower Assembly with Wand and Swing over to the record.
The Vacuum Wand itself should "just touch" the Record.
If it sits too high above the record, or you believe it would drop further than the Record's Surface, take an Allen Wrench of the proper size, loosen the Set Screw slightly on the Delrin Collar-Ring that is around the Vertical Delrin Vacuum Tube and move this Collar up or down as required so that the Velvet Protective Strips just begin to touch the Record.
Re-Tighten Allen Set Screw without moving the Collar. Check again. Repeat if necessary.
Remove Vacuum Wand Assembly, replace Helical Spring, replace Vacuum Wand Assembly, you are done!
Thus, the Spring now has basically no influence of downward pressure of the Wand onto the Record's Surface, that the Spring's basic fuction is only to automatically retract the Wand when the Vacuum Motor is shut off, and it then lifts off the Record due to loss of Suction.
Hope this tweak, and the proper adjustment per Harry W., of the Wand helps!