MK-84:
The first part of your question is easy; the second is near-impossible.
From what I can tell from multiple references, virtually all TBM-3's and TBF-1C's had eight rocket hardpoints. These varied from Mk4 to Mk5 ("Zero Length") rocket rails, depending on aircraft type. Virtually all TBM-3's had Mk5 rails. There's a big difference between the rail types - the MK4 rails (think AH Typhoon) reduced the Avenger's top speed by 17 knots, but the MKV rails trimmed that "greatly". ("TBM/TBF Avenger in Action", Aircraft Number 82, Squadron/Signal Publications, 1987). My guesstimate is the speed penalty with 8 5" HVAR's on Mk5 rails is 12 mph.
Regarding "how often was this configuration used?" - that's the tough question. With one or two exceptions, all the period photographs I've seen show when rockets were mounted all eight were loaded. I suspect the choice of 6 or 8 was mission-specific and only those missions requiring every nautical mile of range called for the use of six rockets.
Almost certainly all the CVE (Escort Carrier) operations in the North Atlantic involved the use of TBF/TBM aircraft with eight rockets as these were shorter-range, shorter-duration sorties. Details of anti-sub operations involving eight rocket loadouts are available in the reference above and also in "TBF/TBM Avenger Units of World War 2". (Barrett Tillman, Osprey Publishing, 1999)
There is available data on the total number of rockets fired by aircraft type by squadron/time period but I have not found any data on the number of combat sorties by aircraft type by launch loadout.
From the research I am confident that the eight rocket loadout was more commonly used than the six rocket loadout.
