There are several national tuners besides M7 doing heads and you should check them all out with their prices. Remember, WMS head come WITH a CAM

WMS
M7
Pilo
Endyn

Also, depending upon where you live, you got to remember how do you get service? Forget about MINI at that point. If you live 1000 miles from these people its going to be mighty inconvenient if you have problems of any kind. You might want to check local independent racing shops ... Maybe specializing in small 4 bangers or a local BMW/Mercedes shop or even an old-fashioned speed shop/machine shop. After all, people have been blueprinting engines and polishing and porting head long before MINI existed. I would check around locally first to see what is possible ... makes service a lot easier.