I also found Guarder springs to be rather soft for what they're rated at, a SP100 gave me barely 1J on the striker and a SP90 got my old ARP (180mm inner) to barely 0.65/0.7J.
Btw, SP and M ratings are done with different bb weights, M springs are supposed to give you the number (in m/s) on 0.20g bbs, so a M100 means 100 m/s on 0.2s, whereas the SP springs are measured with 0.25s, again a SP100 would give you 100 m/s on 0.25g (around 112 m/s on 0.20s). That's the theory anyway.
Meanwhile a SHS M90 spring in the striker gave me 1.2J easy (300mm inner and a cylinder volume not far from a full v2 one), the same spring also gave me 110 m/s on 0.2s in my specna 933 (3/4 cylinder, 260mm inner).
Most likely a combination of the two.
Also you mentioned that utter POS ProShit hop unit, did you keep the same unit throughout the test? Or did you go and swap it at some point?
IKR, the FCU is the main selling point of the whole line fucking TM recoil syndrome right there