BBB or Tacx jockey wheels have little bearings in them, and are inexpensive. Friction Facts (owned by CeramicSpeed) seem to show that bearings are better than bushings, which I think you get on all Shimano RDs except DuraAce.
@Yellow Saddle has separately suggested that a bushing works better for the top (guide) wheel to allow chain/sprocket accurate alignment.
Once my OEM 105 jockey wheels (@ ?15,000km) wore out I replaced them with
Tacx ones from my friendly LBS and before installation, on their advice, prized off the little plastic shield (pops off) to insert some light grease on the bearings (and then popped the shield back in (obv)). When they get an occasional (?5000km) deep clean I repeat this.
Losses in jockey wheels (which are small anyway) depend on bearing/bushing performance, chain return span tension (which itself depends on cage spring tension/design) and chain articulation (inversely depends on number of teeth /size of jockey wheel). The first is the most 'important' wrt power loss but the last is important 'cos it's only by having a noticeably (preferably coloured) large tension jockey wheel that other riders can see how awesome you are / how much effort you've put in to maximise drivetrain efficiency.
