IMI brass is definitely mil-spec brass....................... and damn fine brass at that!
All your numbers mentioned there are spot on for OAL and brass length and the 41.5 gr of 4895 with mil-spec brass has been the standard charge for high power shooting with the M1A/M14 for eons.
Here are 2 good word of advice for loading the M1A:
1. Make sure your primers are installed slightly below the face of the case head.......................high primers can cause a bad day in the form of slamfires.
2. Since you got a new rifle, there should be a card enclosed that tells you what your chamber headspace is............LIVE by that number. Make sure your sized cases are headspaced at least .003" less than that number - again, this will go along way in preventing slamfires.
I've gotten very good wear out of my cases with my M1A, but I anneal the brass after 3 loadings and I think that helps case life tremendously.
As far as using commercial brass, rather than mil-spec brass..................the old NRA formula calls for reducing the charge by 12% when going from a comercial case to a mil-spec case. If you weigh your batches of brass, you can reduce by the percentage difference between the different batches weights............................. but always make safety a 1st priority and work up to a new load.
Also watch what powders you decide to use if you venture off from 4895........the gas system doesn't play to well if you go with too fast or too slow of powders.................4895, 4064, Win748, Hod BL-(C)2 and those other powders in that same range are safe to use. Varget is questionable - I've heard and read good and bad things, so be careful with it.
Here is a great link to everything M14/M1A:
http://www.zediker.com/downloads/m14.html