It's worse for Cure I than that.
Cure I and Cure II each take up the exact same amount of uptime, as all sub-GCD spells do: They consume... a GCD each.
A cast-time lower than the GCD only allows for more movement, not more uptime.
Pre-SpS, 3 Cure I casts will take 7.5 seconds. 2 Cure II casts will take 5 seconds. (Rather than 4.5s vs. 4s.)
Though I'm also not sure where these MP values are coming from. Cure I costs 400 MP each, while Cure II costs 1000 each. So it'd still be 800 less MP for 3 C1s than 2 C2s.