Hate to sound like a honda fanboy on a mazda forum, but what the hell are you talking about? t-belt/waterpump replacement is similar.
..and since when are hondas engines considered unreliable? maybe I haven't researched deep enough into miatas, but I've yet to come across 120hp/liter+ N/A miatas revving to 10k+.
not really sure what you mean by missed downshifts either, are you reffering to weak synchros?
/rant