300 grams off is more that quite a bit; it is a ton. And no, changing just the pistons will result in a vibration too, just not as bad as what you have; a 340 case showed up here within the last year where the pistons alone were changed to KB's and the bobweight was something like 170 grams off; IIRC a 'friend' said it would be OK LOL. So, yes these things happen.