These are my observations and experience regards brake pads.
Everyone will not get the same wear mileage, it will depend how you ride and where you ride.
So it depends on how 'you' ride or have been trained to ride and if you do a lot of long distance, fully loaded riding or commuting in traffic or combinations of each, basically how much and how you use your brakes!
What I have found is that different brands and compounds of pads also have a big effect on the wear milage.
I can base this mainly on that I consistently commuted all year round for ten years on two 883 R Sportsters (05 & 09), the same journey 25 miles each way, around 17 miles motorway/dual carriageway, the remaining heavy traffic & filtering along with similar experiences on our other H-Ds.
These bikes all being in the main fully serviced by myself.
Regards the Sportsters, I would get around 12K miles from the rear OEM pads and around 25k miles front
Tried EBC organic and this dropped to way below 8K miles on the rear and loads of brake dust and not such good braking on the front & changed before they wore out.
Tried EBC sintered better milage but still only around 10K miles on the rear and not quite the same front performance on the front but also noticeable wear on the disc and still more brake dust than OEM but less than the organic.
So regards the Sportsters I reverted back to OEM with the similar milage and just looked out for cheap deals such as 20% off and new old stock.
I always keep a full set of pads in the draw to cover each of our bikes so can afford to wait for a deal!
I've tried a lot of EBC pads on various models but have always found for me they don't give the same wear milage, so are not quite as good value as they first appear, but the deal breaker has often been the dust, especially on the rear wheel with white wall tyres and some of the sintered compounds do wear the discs quicker!
I've tried various others including Lindal which did work well with little dust but were hard to get and expensive. When I ran Harrison Billet six/four calipers (Brembo P5 Pad) on various bikes I found Ferodo sintered pads worked best with the stainless aftermarket discs, but would eat standard discs.
It's all a balance of finding what is right for you, these days most pads other than fake or non-branded e-bays cheap ones will work well, but you may get far more dust, wear and or wear the discs in the case of heavily sintered material and don't be tempted to use 'Race' pads as these generally will not work properly in road use.
Also bear in mind that often the OEM front and rear pads are specified of different compounds even when they are the same pad and the OEMs are supplied with replacement pad pins and clips where as a lot of the alternatives are not!
Regards the M8s, as we have with white walls, I have only used OEM, however Brembo are now retailing pads for late model H-Ds and I will certainly give these a go as Brembo supply the calipers!
Brembo Catalogue Link