I can't find 10Gb SFP+ transceivers supporting cable length over 300mt (we need 400/500mt) is there a standard/physical limit on this, or such transceivers, with extended length support, do exist? We're using a OM4 cable...
If you have not already bought and installed the cable then you should install single mode fiber instead and use LR transcievers. This is the officially supported way to do links of that distance at 10G (and beyond).
If you already have the cable installed then you may as well give a pair of LR transceivers a go. AIUI while primerally designed for singlemode they will in practice often work better than SR on long multimode runs. https://www.flexoptix.net/en/blog/2011/09/getting-a-10g-stable-ethernet-link-even-when-using-old-multimode-fiber-om2-om1/ claims they were able to get a link that passed bit error rate tests down 640M of old fiber, so 500m of modern multimode fiber sounds like it would have a pretty good shot of working.