The IRST driver you downloaded is correct. Your board has the P67 chipset, which is one of the Intel 6 series chipsets. If the IRST version you downloaded was not correct for your chipset, the installation would stop when it detected the incompatibility.
Your AS SSD results are in general fine for a 520, using the msahci driver. The type of data used by AS SSD for its tests, all incompressible data, will always result in lower AS SSD scores with SSDs that use the SandForce SSD controller, as the 520 does. The majority of data a PC user uses, such as Windows OS files, is compressible. The main specs for the 520 are stated using the ATTO benchmark, which does not use incompressible data. Intel does include specs in the 520's detailed specifications, when it is tested with incompressible data. The 480GB 520 in that case is rated for 500MB/s Read, and 235MB/s Write speeds, and your 520 does better than that.
Here are the detailed specs: http://www.intel.com/content/www/us/en/solid-state-drives/ssd-520-specification.html
The IRST driver usually provides better performance.
Frankly, I have noticed over time that there is very little correlation between AS SSD scores and real world performance, for SATA 6Gb/s SSDs that are installed correctly. I've used SSDs that score much higher in AS SSD than the 520, and IMO the 520 performs better in actual use.