Vithavouri makes very good powder in my opinion. Never encountered any problems with them during my 15 years as a reloader. Im not an extensive reloader but it is the most commonly used powder here besides Normas so I take my buddies experiences with me here.
Never loaded a 243 but 7mmm rem mag and 6,5x55 works the N-165 just fine.

Uhmm just noticing that the recommended starting load in .243 win and a100 gr hornady bullet is 44 grains N-165 and the maximum is 49,2 according to their data. But Ive only made a quick check so you had better looked yourself.You probably have it already but here is their datahttp://www.lapua.com/

Tell how it works out
3D