not sure where you get that the connect-X2 cards are not well supported? :-/
if you're using only HDDs, the SAS2008 chipset based cards are just fine. you really won't benefit much from any upgrade of the HBA; unless you're doing really high IOPS on HDDs; which would seem like not the best match.
if you're using SATA-III or SAS-2 SSDs (6Gbps), and have a PCI-E 3.0 capable motherboard/CPU combo, then the HBAs based on the SAS2308 chipset might be a meaningful upgrade. it will consume a few more watts though, as the chip on the 2308 runs faster i believe.
if you're thinking about a SAS3 HBA like SAS3008 and above, you should be thinking one of the following:
1) you plan to use SAS3 SSDs, or
2) you plan to use SAS3 HDDs that only work with SAS3 HBAs (like some Seagates)
The SAS3 HBAs, oldest being the SAS3008 are usually significantly more expensive than the 2308 or 2008; so you'd have to decide if it is worth the expense.
Some times legacy is good... in the case of FreeNAS, the driver for 2008/2308 is very mature and stable and because it is so broadly used by the community, if you ever ask for help about it you'll have plenty of people that can share their opinion. If you use something really modern, say like SAS3408 based controller, you'd probably hear mostly crickets from the community.