A couple of things. First, you will typically lose to about 25 fps/in with the .308 in the barrel length range you are in. So your 20" tube should be about 100 fps slower than the 24" SAAMI standard test barrels produce, assuming your chamber is as tight as those are.
I converted the Hornady G1 BC of 0.53 to a G7 BC of 0.263 at 3000 fps and then adjusted it to 0.258 based on typical changes with velocity and ran the tables. The G7 tracks changes in BC with velocity better for this bullet shape. I get about Mach 0.97 at 1000 yards with your MV with that. Check Litz's book for more accurate measurements of BC at different velocities. But that's only if I believe your velocities. Based on Accurate's data for a 24" barrel with a Winchester case, dropping the water capacity a couple of grains for your Lapua brass, I get about 2345 fps from your barrel. You could have a "fast barrel" with a tight bore or other factors involved, but I would double-check the readings over a second chronograph to be sure there isn't a measurement error getting in somewhere. Many chronographs are only good to within 50 fps or so anyway, and maybe that's the issue. I've not used a Magnetospeed, so I don't have a sense of how well it does with that short sensor spacing. I'm not knocking it, I just think it sounds optimistic. If my velocity guess is closer to absolute, then your 900 yard speed would be about Mach 0.98 and 1000 yards would be at about Mach 0.93.
If you are going to shoot long range with a short barrel, you will have to pick and choose bullets carefully. The original 168 grain SMK is almost certain to be a bad choice for anything much over 600 yards. Bryan Litz points out it has a dynamic instability that becomes apparent below about 1400 fps (top of the transonic range). I have personally witnessed them tumbling and haring off every which way on a 750 yard popper, and when I attended Mid Tompkins's Long Range Firing School at Camp Perry in 2001, almost everyone on the line shooting 30 call had 168's and nobody could stay on the 800 yard targets with them and the pits reported keyholes for the ones that did hit. They are great out to 600 yards, but my advice is don't bother messing with them beyond that range. I have no information on how the new secant ogive STMK (Sierra Tipped MatchKing) bullets compare. If those are what you got, I don't know what to predict. I can tell you the 175 grain standard MatchKing does not have the instability problem. This is due, at least in part, to it having copied the 9° boat tail angle empirically arrived at by the Army after WWI for the M1 Ball bullet, rather than the 13° on the original 168 grain SMK.
Kevin Thomas was still a Sierra ballistics tech at the time and attended the same Long Range Firing School I did. When everyone complained about the keyholes, he said the 168 was intended as a 300 meter International Match bullet when it was designed, and the fact it worked well at 600 yards was a bonus for Sierra, but it was never meant to be a long range bullet. He recommended the 175 for long range, and at the lunch break at the school, everyone with 168's ran out and got ammo loaded with those bullets on Commercial Row. After lunch, no more keyholes or other problems. The 175 sails right through the transonic range. If you want to get past 600 yards, I can recommend that bullet. Again, the new tipped version I have no information about at this time.
The 155 grain were developed for .308 Palma match guns which typically have barrels of about 30 inch length to get enough velocity to keep the bullets supersonic all the way to 1000 yards. They use a 9° boattail. Their BC is good at muzzle velocity, but it drops off more than the 175's do when they slow down, IIRC. In other words, they can be a little more vulnerable to wind deflection. They were designed for a rule-imposed weight limit for Palma, and not because that weight is the best ballistic choice. They also have a smaller bearing surface and it pays to seat them with as little runout as possible. Search the forum on the topic and you will find some information on seating methods and on sizing cases to minimize pulling the neck off axis with an expander.
I recommend you look at
Dan Newberry's OCW load development method.
