It's probably best to stop there, otherwise you're in weight weenies territory cutting as much grams as much as humanely possible while you still weight the same as before.
lose weight on your body, you'll get a significant performance boost than a slightly lighter stem.
Any recommendations on lighter/better tyres? I was thinking of GP4000s, which are better but not really much lighter.
Build so far:
Cannondale Caad 10 48mm
Ritchey WCS carbon evocurve
Cannondale c2 seatpost
Pro PLT -10 degree stem 90mm
Duraace 7900 shifters, FD, RD, brakes
FSA slk light carbon compact bb30 crankset
Duraace 11-28 cassette
Ultegra chain
Arione R3 saddle
Zipp team csc Alu wheels
Gatorskins 23mm (to be replaced by gp4000 when they wear out)
Planet X lightweight inner tubes
60g Zipp aero skewers
Bontrager XXX bottle cages x 2
Jagwire road pro cables
Look keo 2 max carbon pedals