Miracle seed and grass plate both boost the same amount of power , which is boost 1.2x the normal damage
Ex. grass type move with 50 damage, powered up by seed/plate, become 50 x 1.2 = 60 base damage
And all of the type-booster item will always boost 20% more from the base damage or 1.2x the base damage, even with the incenses (type-booster only)
BUT with the gems you can find in dusts in any cave in pokemon black & white, you can get a 50% boost power!!! Well, as you know, gems can only used once, so if you use the gems, choose your move carefully...