More answers for others that may search one day:
Yes, spring rubbers or "helpers" can be of real value. But remember, these are coilovers, and the spacing between the shock body is pretty minimal. Personally I think the best option is an insert that follows the wrap of the coil, and that it be placed at the bottom the coil. I'm thinking at the same place as the shaft bump stop.
If there was enough space at the shock body at the spacer this too might work. Seems likely there isn't enough room there though, but it would sure be sweet if it did. Maybe a tie-wrap would help.
So the math behind this turn out to be pretty simply: Rate is the reciprocal of the effective coils.
If we cut a spring in half we all know it would double the rate. The reciprocal of 1/2 is 2/1, with simplifies to X2. Perfect, it seems the math works. So now an example pertinent to the XPT:
Lets are an 8 coils spring rated at 225lbs. Lets block out one coil, leaving 7 effective, and multiple the initial rate by the new reciprocal of 8/7. Lets call that 260 (It's 257 and change). 1.5 coil turns would be 277. Darn, I like that rate, and I hope this works.
Spring spacers at Farm and Fleet are $6 each.
Full disclosure: Spring rubbers have their own rate unless they are rigid. This can affect the results, and in racing circles rubbers are rate in pounds themselves. Given the option I would be taking "firm", or "hard" rubbers so I was getting about what I expected.
As with all these posts, I hope this helps others as they consider their spring decisions, and I hope you reward Eibach and Walker with your business, for they're the ones openly telling us what they recommend.
Props to all,