The mentioned gains are achievable without loses with the ability to dynamically adjust the timing, the little and already very cheap as second hand device Bipes Acu does this on MX5 1.6 and 1.8 engines. Together with the "crystal mod" the following gains are easy achievable on normal fuel and flawlessly presented from Loren on miata.net:
http://forum.miata.net/vb/showthread.php?t=186399