Another week, another new SSD low.
$399.99 Makes it about $0.78 per GB. A few months ago I was struggling to find anything under $1.50 a GB.
I purchased a few Hitachi 3tb drives from B&H last time they were on special and had no issues. Received within a week using cheapest shipping option.
I do not think the M4 can saturate SATA 2 for there to be any loss.
You will be effectively limiting yourself to 3Gbps. But the M4 doesn't quite rich that limit. An 6Gbps controller will have little if any affect. So go for it.
http://www.anandtech.com/bench/Product/355?vs=560