Nicely done. Looks like your group was 3MOA or so in size; much bigger than your deeply-sub-MOA group at 960y. BTW shooting sub-MOA at 1000y is a worthy accomplishment of any target rifle. It's quite unusual and remarkable to get this out of a factory gun with a factory barrel, well done!
You probably already know this but the Sierra 168 is a relatively poor choice for shooting beyond 800y (though at your elevation of 4500' you can probably get good results at distances a bit further than that, such as your 960y shooting demonstrated). They have a dynamic instability and at some point downrange (typically between 900 and 1000 yards, though it does vary a bit depending on muzzle velocity and air density) they will start to tumble. This has nothing to do with going subsonic, though that is the most common explanation you'll hear.
If the 155 Palma bullets that you have on hand shoot accurately for you (say, half-MOA or close to it at 100y or 200y), you'll probably get much better results with them at 1000y+ that with the SMK168s. When doing your short range load development and testing, try to get your velocity standard deviations below 15fps (ES below 45) before taking them out to shoot at long range (this is to reduce the vertical stringing caused by slower vs. faster bullets in the same string)
You might also find it worthwhile to try one of the heavier high-performance target bullets. The Berger 185 LRBT jumps to mind as an obvious choice, but there are other good choices too. Factory Remington chambers tend to be very long throated, so the longer (and further-forward) bearing surface of longer and heavier match bullets will let you get your bullet closer to the rifling and also take best advantage of your case capacity. Also your (relatively) short barrelled rifle will be slightly less penalized with heavier bullets than with lighter bullets, in comparison to ~30" barrels.
If you are shopping around for other bullets to try, choose ones with a reputation for being "easy" to tune, i.e. they are relatively insensitive to seating depth. The SMK168 and the SMK175 are quite good in this department. The new SMK155 (part #2156) is a bit fussier. The Lapua Scenar 155 and the Berger 155 VLDs both tend to be quite fussy and should be avoided for this reason especially if you have a longer throat (though both of these are superb top-quality bullets). The Berger 155.5 Fullbore and 185 LRBT are quite "unfussy" bullets. The one time I tried the 155 Hybrid it seemed "unfussy". Haven't heard yet if the 185 Hybrids are unfussy or not.
Keep up the good shooting and documenting. What nice country you have to shoot in.
