After couple days of testing i could state that with DC32 firmware did not fix all issues.
it still shows "prohibited" sign at startup for a few seconds sometimes, which, i guess, means "waiting for boot device". After that it boots more than a minute. Macbook periodically stalls for 5-10 seconds, not so often as with DC12 firmware, but these hangs are very annoying.
I did all suggested tasks - reset NVRAM/SMC, run extended Apple hardware test with no errors shown. There are no suspicious records in the system log. Is there anything we can do to help you pin down remaining issues?
Macbook 8,1 (13-inch mbp late 2011) with INTEL SSDSC2BW240A4