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It would certainly explain why some m4s/c300s are giving way lower 4k results than others.
It's possible that as-ssd is pretty cpu intensive on some tests and it might only be using 1 core like most programs do so the extra core clock helps a lot in those situations.
c300 is faster in benchmarks and real life unless you're looking for purely sequential reads/writes.
That being said the m4 is a better value since it's quite a bit cheaper.
Here my 64GB c300 which is almost completely full.
I've seen a wide range of 4k results for the new m4 ssds however the result I found on overclock.net below doesn't look too far off from a c300.
Why not get an xeon cpu it's not like they cost much starting at $210...
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100008494%20600126647&IsNodeId=1&bop=And&Order=PRICE&PageSize=20
Some boards will not work properly with hardware raid controllers.
If you want to be on the safe side stick with a server board like Tyan or Supermicro both make 1155 socket boards at reasonable prices.
RHEL6 kernel probably has it backported in just like they do with many other things.
It would still be a good idea to compile the latest source code driver however or at least take a look at the one they provided.
Few more things to check are you running the latest firmware? Also, check to...