Is it worth to get a SATA-3 controller to max my SSD out?

I have an OCZ Agility 3 on a Gygabyte GA-P55-UD5 (it only has native SATA-2 support, not SATA-3).

I have to admit performance is already interesting but should be half of total possibilities. I've discussed about this with my hardware vendor, and he said that it would be almost useless to upgrade my SATA controller to SATA-3 because I would hardly notice the difference. Is it technically right?

How can a peripherial/device work at twice its speed and go unnoticed?

I would say, without a doubt, yes. SATA-III has a maximum throughput of 6.0 Gb/s, while SATA-II has a maximum throughput of 3.0 Gb/s. If we take into account the fact that both use 8b/10b encoding, both buses max out at a reasonable 600 MB/s and 300 MB/s.

I personally have the 120 GB model of the OCZ Agility 3, and it is running on my motherboard's native SATA-III port. During benchmarking, I hit well over 500 MB/s actual transfer rate, so you would be limiting your maximum transfer speed with the SATA-II controller.

If you do wish to upgrade with a new PCIe SATA-III controller, please see my answer to this question, titled "Adding a SATA 6 Gbit/s PCI Express controller card?". Summarizing what I wrote there, let us assume you use a worst-case PCIe 1x controller card. Your maximum throughput would be roughly 450 MB/s, or 1.5 times what you have now.

Whether or not you notice the difference in those transfer rates depends on you as a user, and what your purposes for the drive are. Even if your drive was capped-out at 300 MB/s, it still has nearly zero seek time compared to a conventional mechanical hard disk, and that's what most users will primarily notice.

After the whole seek time debate (because lower is always better), it depends on what you have stored on the drive and what you use it for (e.g. bandwidth intensive tasks like video editing, or low bandwidth tasks like internet surfing). That itself will dictate whether or not the increased bandwidth is warranted in your case.

Depending on your budget did you ever consider getting a PCIe SSD solution like the OCZ RevoDrive3? The costs for the 240GB version look highly competitive with a 2.5" form factor and there's no SATA protocol versions to worry about... as long as you have a full-height PCIe slot, you can get 130k IOPS.