Yes, only gmapsupp.img will work in the Garmin folder in SD. You can create a 'Map' folder (no quotes) in the sd root and put many maps in there and use 'anyname'.img eg CNNT_SG/MY_2012.40.img or anything else convenient.
Yes, thanks. I see from your post yesterday that you downloaded the Type3 pack. Same map info as my last Type2 dl, makes sense. Robinang reported a header date of 10.02.2012 which must mean different map info in at least one other pack type (1,4 or 5) for some reason.
Comparison is not always subjective when only one area is examined as you realize i'm sure. Simple answer, have both and use whichever seems best where you are at the time. Don't select both maps together of course or there will be problems. (Go to: Tools>Settings>Map>Info>Check or Un-check as req'd). On the other hand, the two different jcv files in the 'JCV' folder should not cause any problems.
Size could be also to do with differing compression rates used by the mapmakers, not just the map data. Higher compression rates may also explain slower drawing and/or searching.