I've spent a ton of time with the HDO microfiber and the standard LC microfiber cutting pads and IMO they are virtually identical except for the tapered edge and center cooling chamber on the HDO microfiber cutting pad.
Now all I buy are the regular LC microfiber cutting pads and I punch the center cooling chamber out myself. I honestly never realized how poor the quality of Megs microfiber cutting pads were until I started using these LC microfiber cutting pads.
I don't own a 21mm long throw polisher but the standard LC MF cutting pads seem to stand up to the added abuse and heat of a 15mm long throw polisher just fine.
I don't think the additional costs for the HDO microfiber cutting pads is really necessary.
You should definitely try using getting a 5/8" hollow punch off amazon and punch out the center cooling chamber yourself. I think they're like $10-12 and you don't even have to use a hammer to punch them out, you can just twist the punch and it slices right through the pad. It has really helped increase the life of the foam interface on both my megs and LC microfiber cutting pads. My megs MF don't warp as much as they did before punching out the center and the foam interface and velcro backing now last longer than the actual microfiber nap