what model cm16 ???
Long 357 or short 230 barrel
barrel lengths and the cm16 range varies greatly so for anybody to even "guess" the spring will be a bit stab in the dark tbh
normally Element m105 to shs m110 on shorter barrels
AoE drops fps by about 10-15fps
but bearing spring guide pulls you back by adding 15fps aprox
piston head is $hit seal - never got a great seal with stock G&G piston head myself
nozzle shs red m4 but might be a whisker too long for some guns/boxes
(it is not as easy as it looks at first - there's loads of stuff to check when upgrading/servicing the box)
Long barrel m105
short barrel m110
that is my guess but depending on gun it will only be a rough guess until chrono and depends greatly on seals everywhere
If it is still down you can add a bearing spring guide as I said
heck if you wanted just pop in a bearing spring guide as is to boost 15fps
but without more work AoE etc... I wouldn't go mental on the juice or change motor
it really is a question how far you wish to go
and fps means little if shooting a bit crap - no spring will change that - it is hop/bucking/barrel as said before
tbb may up your fps by 10 or so anyway