to fix it, buy a reseal kit for the sunroof, new water pump should solve the overheating i would believe (its a water cooled car after all)
performance - FR wilks adaptor board and power prom
http://www.frwilk.com/944dme/ headers and exhaust.
that would probably solve your speed umpf
my924gtc and x3m944 are working on a supercharger, that should def. scratch your itch if the rest didnt
oh ya... and before i forget
check your belts