My son has an HR215K1 SX masters series, and I have an HR215 SX, and last year I installed the mulching blade setup onto both of them. Only the Masters series had the mulching plug installed and I found that the HR215 w/o the mulching plug didn't work as well if I tried to mulch w/o the plug, so I obtained a mulching plug for that one too. If you have the mulching blade setup installed but don't want to mulch, there's no problem because you just attach the grass bag. The mulching blade setup cuts the grass into smaller pieces which fit into the bag more efficiently, so you can cut more grass before emptying the bag.
With 2 different HR215 mowers, I have not noticed a drop-off of power with the mulching blades installed whether mulching or bagging. There might actually be a very small power loss, but I haven't noticed any loss to worry about. If you're experiencing noticeable power loss, I suggest you check every aspect of the mower which could lower power output if not up to specs. For example, I picked up a used HS520 SB a few years ago, and it seemed to run fine, but I noticed that it didn't have the correct spark plug installed, so I replaced the spark plug with the correct one, and the rpm jumped up a few hundred rpms.