Last night using 12" lightbridge dob and 8mm Delos in a ParaCorr Leo I was visible in mag 21.5 skies at meridian. At 1753mm focal length the 8mm Delos offers 219x with a 27' FOV so regulus could be 10' outside of the field with Leo I just a bit off of center.

The slightly blotchy large object elongated perhaps 1.3 or more along E-W had non-distinct edge definition but was unmistakably present with movement of the scope.

I would have to agree that to see this in 6" or less would require the best of transparency and very good eyes on a very dark sky. I may try that in my 18" this season as I have a 6.2" primary mask I use sometimes for planetary and seeing estimations. (One of the key 'official' seeing estimation scales for observatories is with 6" refractor). My eyes may not be able to see Leo I with 6" is my guess but I do know people who have 1/2 or better 'limited mag' and they may just be able to see it in 6" ideal conditions.