I just did the subframe bushings and my initial impressions are exactly like yours. Its a mod that i waited to long to do. The labor involved sways a lot of people from this instal but it is worth every penny. That rear end bounce and 'jiggle' when you floor it have been eliminated. The car feels much tighter.
MY Build Log
08 AW 135i | Cobb | ETS FMIC | ER CP w/ Tial Bov | AR Catted Downpipes | Berk Mids | Meisterschaft GT SUS 1M |M3 Control Arms | M3 Sways | M3 Rear Subframe Bushings | Wavetrac LSD | AST 4200 DA Coilovers | HRE R43 | Hankook R-S3 |Full BMW Performance Interior | Revozport 1M Raze Aero Package in Satin Carbon |1M Body Conversion|