Races in Heroes 5.

It won't be higher than that of a Necromancer or a Warlock; they have a 45% chance of getting a bonus to spell power. A Ranger gets 30% to knowledge and 10% to spell power, so in total, it's 40%, which is lower on average than those two races.

Strictly speaking, 5% isn't a significant number. At level 21, it's just +1 to one stat. It's easily compensated for by any bonus to knowledge, an artifact that boosts knowledge, or (even more so) education. The question is whether you should rely heavily on spell power from Druids; it's nice and all, but it's not entirely equivalent to pure spell power.