I just tried this on my sparkletop.
I have a damper/hammer assembly and harp supports scavenged from a Mark I. I measured the height of the harp supports and found that the bass end was 1/4" taller.
On the sparkletop there was only a 1/8" height difference on the harp supports. So I experimented with shims on the bass end only.
First try was a 9.5mm shim and hammers would not strike the tines.
I then tried a 6mm and the hammers worked although the dampers need to be adjusted. It did have a positive impact on the sound. It resolved one of my complaints about sparkletops - the bass notes don't have the "furry" tone of the later pianos. This shim brought out that "furry" sound - I *LOVE* it.
One big improvement is I don't have to play very hard to get that furry sound, so a lighter touch means a lower risk of breaking the unobtainium Raymac tines, especially the longer ones for bass notes. The other improvement is it eliminated the "wolfy" tone of some of the bass notes, now they ring pure.
The middle notes had a slight change in sound, the top notes sounded no different. The top 2/3 was my favorite area on the sparkletop, now with the shim the entire range is optimal.
I'm making this shim permanent.