HC is your problem lol. HC is known for alot slower paced than core. That is the reason I play CoD is for faster paced than BF3 so GW is my main choice or domination for maximum score, and I have a 512 spm with 2.75kd and a 1.7 w/l playing solo.
Depends on gametype.
TDM will be lower than Domination or Hardpoint, as there is less opportunity to earn score.
If you're playing TDM solely, I'd say anything over 250 is good.
If you're playing Domination, I'd say anything over 400 is good.
If you're playing Hardpoint, I'd say anything over 500 is good.
If you're playing Kill Confirmed, I'd say anything over 550 is good.
That being said, there are different playstyles that can be equally team-based and effective, but will result in different scores. For instance, a flag-capper will get the 200 points, but the guy defending him while he caps the flag won't get any points. The guy controlling half the map in Hardpoint, not allowing the enemy team to pass, won't get as many points as the guy who holds down the Hardpoint location.
SPM is a flawed stat.
I usually use LMGs and sometimes SRs so if I get the highest SPM in a lobby then something's wrong.