I have not purchased a 5th for spare yet, but yesterday I removed the OEM spare and took some measurements. It appears a full diameter donut will fit in place, allowing the stock storage compartments to remain. I also believe a full 225/65R17 if I remove the upper foam storage caddy. I have not decided which route Im going, but Im definitely going to replace the OEM spare with a proper diamater tire.
Re: Speedo - Surprisingly I found the 65-75mph range to be spot on using a GPS for comparison. Perhaps the factory config was at the outer limit, and the .9" increase in diameter corrected this?