Okay, so... a few questions:
1. What is the application?
2. You want a subsonic 243, correct?
3. Will you be shooting suppressed?
4. What is the barrel twist rate?
I have a 243, love it, but push it faster rather than slower. My fist concern will be twist rate to stabilize the bullet for subsonic use.
In general, since you are not looking for velocity, the larger the bullet, the better (since 1/2 mass x velocity squared gives you the kinetic energy, and the v is set, making the m bigger makes the KE bigger.)
And if you aren't running suppressed, it is mostly a wasted adventure. Subsonic is much louder than in movies, even suppressed it is not hollywood quiet most of the time.
In the end, I am still wondering about application.
Lastly, it looks like that rifle is a semi-auto, I know nothing of the action but do know that taking an action meant for supersonic ammo and putting subsonic through it often results in failures to extract, eject, feed, and generally function reliably. Subsonic rounds produce far less gas pressure than do their supersonic counterparts, and the actions often don't have enough pressure to operate correctly.
In short, the whole thing might be fraught with peril, it might be worth doing, but it probably won't be nearly as easy as you might think.