Not sure what you mean by 2.8mm HO track--2.8mm would be about the equivalent of Code 110, whereas 2.5mm is roughly Code 100 and 2.1 is Code 83 (measurements in thousandths of an inch)?
I haven't seen any code higher than 100 used for HO track--it would be the equivalent of 220-250 pound rail, heavier than anything used on the prototype.
It should be possible to modify a standard Code 100 (2.5mm) joiner to meet the larger track--or you could solder a piece of 2.5mm to it while sitting on a .3mm shim to match heights.