I'm not sure if 1:8" twists will stabilize 55 gr. projectiles, but I'd get the rifle. There are a great many heavier projectiles out there which will work wonderfully, and you'll like the final results.
Tikka T3 seems to have a 570mm barrel which is @ 22.44 inches. You would be on the edge of 55 grain with 8 twist.
true 223 runs at lower velocity than say a wylde or 5.56 higher pressure rounds. I would say to shoot it and if the paper holes are not round and it shoots all over the paper decrease the FPS to where the groups become tiny. I got rid of a 20 inch 5.56 7 twist AR because it was worthless with 55 grain because the added expense of having to use 75 grain match type bullets was not worth it to me. I loaded 55 grain down to like 2800 fps and it started to group, but that defeated the intended purpose.
I shoot full powered 5.56 55gr out of 18 inch 8 twist just about 3100 fps. so you could also try that fps