MWJB 58.5mm is probably as large as is practical to go, unless you are going to match tampers to specific baskets.
Based upon my measurements of my 15g and 18g IMS baskets, I can very safely go to 58.5mm. I could probably even go to 58.6mm and might even squeeze in at 58.7, if that was available. But I don’t want to create a vacuum effect.
I can buy an additional base for my EazyTamp for $60 AU per base, plus about $30 AU shipping. So I could buy a 58.5 base, or both a 58.5 and a 58.6. They offer both as accessories.
The real question is: will adding 0.1mm in tamper base make an incremental difference? Vs adding 0.2mm? If either did, I’m all in, as every incremental improvement in the dosing and tamping process can (in total) add up to a significant improvement. If not, I’ll save the $ for other items, like a Lelit B.