it shouldnt vibe more- try a search- i remember there was someone that put one on a rm250 around that year and the same thing happened- turns out that the RMs were coming with the threads on the crank machined a bit off angle wise!
The following quote is direct from the instruction sheet for a Steahly FWW that I purchased but haven't put on yet:
"First be sure that it is actually vibrating more than it did before, you are probably just more conscious of vibration. If you are sure that it is vibrating more than it did before check your main bearings for wear and your crankshaft for a bent or damaged end. If everything checks out you may need to send the weight in for inspection."
You have a Steahly weight so why not give them a call? Their number in OR is (800) 800-2363.
My cr250 has a fresh bottem end and i installed a steahly fww on my bike last week (14oz) i have not used the bike yet but when i started her i also found that she vibrated/ made a humbing noise similar to that of a gasgas ece250 enduro model....i was also a little worried...but since its all enduro bikes which make that humbing noise i think its just due to the heavier fw!