I've not messed with much Blue Dot in the .40, I used it some in the 10mm and didn't really care for it. I'm a little more familiar with Longshot, it's good powder although it doesn't seem to meet it's velocity claims.
10gr Longshot w/155gr Rem JHP averaged 1220 from a 3" Kahr CM40. My guess is that you'll be over 1250 from the G23 Gen4, as I've not tried those out of my Gen4 G23.
If you can, get your hands on some Power Pistol too. Works very well in both .40 and 10mm and seems more consistent than Longshot as well, plus it lives up to it's velocity claims and then some at times. From the 3" Kahr, 9grs with the same Rem 155gr JHP averaged 1200 fps, which would still likely be close to or better than 1250 from the G23.