I think the R56 ones are a little better than the R53, they look to have more rubber in them. And yes the brackets from R53 and R56 are different. But the diameter of the bushing is the same size so the Powerflex will work in both R53 and R56.
I wish OEM castings would be made using harder durometer rubber, ala the Group N homologation parts for Subaru and Mitsu rally-based platforms. Makes it very easy to get replacement bushings but a bit more enthusiast minded in design without all the drawbacks of PU.
BMW offers Group N parts for their M3's, but the prices are ridiculous. Examples: BMW stock E30 rear subframe mount bushings: $54 (turner) BMW Group N E30 rear subframe mount bushings:$422 (turner) Powerflex E30 rear subframe mount bushings:$105 (bimmerworld) BMW stock E46 M3 rear control arms: $70 BMW Group N E46 M3 rear control arms: $205 Now they call it BMW Motorsports instead of Group N.