Anyone know? As obviously walking 30 seconds from bts cant be beat.

You get slightly better rates at their Pratunam headquarters across from Central World. That's where they do a bulk of their business. The small kiosks at Prom Phong and Chidlom are more for small transactions by tourists. You still get a better rate than at the banks.

Their official exchange rate board has been suspiciously broken for a long time now, draw your own conclusions.