To push something like that you will need ALOT of heatsinking and a solid pill, or turn down your amps...I find the XM-L and XM-L2 run very good at 2100mA and don't overheat (even in a no meat at all Convoy S2 or S5), they get warm and throw very good light but when you start approaching the max current you get more heat increase than you do light increase
I also find taking a tiny bit of NOALOX and putting it on the threads of the pill when I screw them in, it helps ALOT in wicking that heat out of the pill into the body, then natural heatsinking
Does the 503b use a P60 dropin [I know my 502B does]? And if it does have you tried the aluminum foil wrap on the dropin to wick heat away...
If your LED emitter isn't bonded to the pill you can either put a thin layer of artic silver, or silicon (yes even that stuff you put on your kitchen sink, or even two part epoxy [remember a very thin layer]) under the aluminum star against the dropin pill (hollow pills you can't bond down like this because there is nothing to bond to), then wrap the pill in aluminum foil so it has a thermal path from the dropin to the body of the host
Per this image on the DX website
The pill is solid, yes, bond down the emitter MCPCB with whatever method you choose, then NOALOX the threads to help wick heat away from the pill to the body, but at 2800mA its going to overheat fast...2100mA might overheat but it will take longer, unless you are willing to tail clicky it to a lower power mode