It is a while ago I tested this for a bit, so maybe some things got tweaked in the meanwhile. I think casting time is already considered by design (not all spells of the same rank have the same practice gain, but slower ones yield more). However, rounding has a big impact, and the practice numbers go down as your rank increases, so often you just get 1 practice per cast. More spell power (% slider) means longer casting times, so yes, this can yield more practice per cast, but depends on rounding/capping.