The VRM heatsinks on your board are definitely on the large side. I personally wouldn't worry about it too much considering the AIO fan will (probably) be either above the IO cluster or above the mobo. There's still going to be a fair amount of air moving across the VRM area. Keep in mind, the faster the CPU runs, the more the VRM is working, but the CPU fan is running faster also = more airflow when VRM is hot.
f0d makes a good point. If you have the ability to monitor VRM temps, definitely keep an eye on them. I don't have an exact target temp for you, but like JBI said, MOSFETs are rated to run at relatively high temps. Probably something that is easily Google-able.
My general rule of thumb: When buying a mobo that is destined for even the slightest (ie stock voltage) overclock, get one with VRM heatsinks of some kind. Especially since rigs of that nature are likely to have a tower HSF or AIO. I even go as far as to favor mobos with VRM heatsinks over those without for ALL my purchases (all else being the same between my choices).
Main: i5-3570K, ASRock Z77 Pro4-M, Asus GTX660 TOP, 120 GB Vertex 3 Max IOPS, 2 TB Samsung EcoGreen F4, 8GB 1600MHz G.Skill @1.25V, Silverstone PS07B
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