Others here can contribute more/better advice than I can, but the following do come to mind:
-how many tries with the 300? Sometimes people do three or four and think that was soooo much but IMO it's not always the case
About 3 cycles, working the product in with moderate pressure, then light pressure
-the MFCP/300 can almost certainly take things down to primer if you do it long enough, but using that MFCP with M105 should up the cut a bit
Tried using UC with the MFCP, similar result, dont currently have any M105
-maybe the RIDS are too deep for safe removal
Very possible, and I dont want to push it too much... Id rather live with a few RIDS then the other possibility... Black 2007 Lexus IS250 is the car... Maybe harder paint??? Im not sure... Dont believe its ever been corrected, so Im "assuming" that the clear is in decent shape still... Dont have a gauge, but from what Ive been reading, even if I did have one that was under $1000 alls it would tell me is the total paint thickness, which IMO is worthless, since I don't have a clue how thick the primer, base, clear were originally.
-I wouldn't switch to the 301 until you have the RIDS as good as you're gonna get 'em