Thanks for your input, folks.
Last night I came across this URL, which puts a positive "spin" on using a Grado Gold with the latest P3-24:
They didn't mention anything about tweaking (spacers, etc.). From what I've read, some of the hum issues reported with the earlier P3 are not to be found with the new model. That would be good news.
From what I've read about the Rega cartridges, I think that, money aside, I would rather use the Grado Gold carts I already own. But the issue is still moot. I hope someone can report success with the Gold and the new P3-24, since one can't always trust an on-line review. What you folks have reported could still be an issue with the new arm and table.
In addition to the cart VTA, I'd probably want to get a different mat. This means raising the arm even more.
BTW, I have a couple of Thorens mats. One is the old ridged one, and the other is a thicker mat. On the Thorens this mat left very little spindle left over for warped records or a clamp. I wonder if this would be true on the P3-24 as well.