My guess is that it’s not poised for a big jump in the near future. I don’t know sales figures for the G37 convertible, but I checked Autotrader and there appear to be 212 of them for sale nationwide, definitely not rare. I think what really pushes cars like RX7 and Supra to such stratospheric values is the extreme amount of performance for their day coupled with very unique styling. NSX and MR2 are helped by their mid-engine configuration.
I think the price trajectory for these hardtop convertibles will be a bit different. I think they’ll bottom out at a much higher point than the sedans but won’t bound back up like the sports cars. If you look at convertibles from the 80s and 90s they do much better than their closed counterparts. While the sedans get mostly crushed the convertibles stay out there and on the market selling for good money. (Take the Chrysler LeBaron, for example). These 2000s retractable hardtops have the added advantage of a solid top and a fascinating top up/down routine. Since so many were made (Lexus, Chrysler, Pontiac, and many others had them) they’ll never be rare, but there will always be plenty of people who want them.
So don’t park it in the barn, but keep it in a cool/dry place when not enjoying it and when you’re ready to sell you should find someone who’ll give you a lot more than the sedan is worth.