I got the $400 9 lb. flywheel from BMP and a stock clutch, PP, TOB. I am totally satisfied with it. I participated in a Track Day event and autox this past weekend (the last, our track is closing

) and I had better times then some M3s and Mini Coopers during Track Day, and won my class in autox.
I realize for some $400 may be a lot for a flywheel that will give you instant performance, but my thought is, a new set of wheels will cost 4x that much and all you get is.....eh a nice wheel!
IMO....the $400 is well worth it, but if you can put together the M20 menu and get all the parts for less, more power to you. Either way, a lightened flywheel is definitely worth the Money and/or Trouble.
As for idling....no problem, chatter too minimal to be bothered, my wipers make more nose.
Do it and be happy.