You will have some smoke with lubed lead bullets. (sorry it to so long to get back to you here)
All the cast lubed lead bullets I have shot had some smoke, they tend to have more as velocity goes up, sometimes not enough to even notice though.
Since I am the only one that responded (surprised) I would be tempted to try a new thread with a heading of something like
HS6 and cast bullets in 9mm
9mm 125gr cast bullets and HS6
etc
Hopefully you will get some more answers/opinions.
Chrono #s would help here to see what kind of vels you are getting but if you don't own a chrono that's out.
I would be tempted to bump up the charge to 5.7 and try some there, maybe some at 5.9 as well. I don't know if HS6 is smokey at on the low end or not.
(maybe more than 5.9 by Hodgdons data, I don't know but most HS6 users say it is happier at the high side of the charge range)
They may or may not shoot better, they may or may not have less smoke.
(not helping a lot here am I
)
For your current 5.5 load
Is the load safe? You should be fine at 5.5gr I would think
Does it function in your firearm? You said no issues there.
Does it shoot well? You said it did.
Sounds like you are good other than some smoke.
If you want to eliminate smoke you can go with plated bullets and that will help a lot.
Slightly more $ but don't forget the RMR 5% discount.
I have had excellent results with plated bullets from RMR.
(nothing wrong with cast I shot 1000 and 1000s of them)