Yes, quality bullets are fine in big bores these days.
Of course, 6000 foot-pounds is easy for a 416Rigby. The impala shown in my post above was that way.
Here is a shot of sighting-in a rifle in the bush, over the bonnet/hood of a vehicle. Before my son and I went out walking and looking for an impala we sighted-in the rifle. I took one shot. Then my son took one shot with the same rifle.
We walked down a hundred yards to inspect the shots.
Well, that is good to go for a hunt. The bullet holes were kissing, though the rough edges don't show that. I dropped the Nikon two clicks for a lower trajectory. The scope tracks well. I don't like bullets rising over 3".
But more to the point, the Rigby handles those loads smooth as silk. The bullet in the same post with the impala picture shows the case with a nice primer and no brass marks on the case head. When cases slide right out they don't have pressure marks. Here is a another supporting picture for this post. It is a bullet from a buffalo, taken with the 416 Rigby at 110 yards. We had already bumped them twice and were able to find the bull grazing in the shadows. The Rigby did the job. An accurate rifle made that shot reliable. the picture shows all the cases used. The first shot, 416, made the buffalo sick, but we found him on his feet standing in a grass patch watching his back trail. I put a second into him from the side, parallelizing much of his body but leaving his head and neck thrashing. My son jumped forward with a 338 and put a bullet behind the head for lights out.
Look carefully at the primers of the Rigby cases on the left. Nice. The 338 may be a tad hotter, a 225TTSX at 2835fps, but still smooth easy bolt lift and reasonable primers. We load to the old book levels of 4000ft#. The 338WM is a light Tikka and is probably as heavy a load as that action should have.
In a modern rifle like a CZ, with modern cases like Hornady, and stable powders like R-17, and a great bullet like the 350gn TTSX, there is no reason not to load up to about 6000ft# (probably just under 60k psi.) Our last three Rigbys have all handled these loads like nobody's business.
That is great and from the target it is quite clear that both you and your son can handle the pressure and the recoil with this load. This is a good combination for the initial shot and also makes for a more versatile application for a client.
Personally I do not like lighter bullets in big bores and especially not for DG.
There seems to always be two schools of thought on this, the high velocity light bullet group and then the old school heavy for caliber at respectable velocity group.
Having to sort out the mess when things go wrong, often at extremely short range, I fall in the latter old school group.
If I owned a 416 Rigby, I would load 410 gr or even better 450 gr bullets @ 2200-2300 fps.
It is bullet momentum and straight line penetration from a premium grade bullet fired at respectable velocity that sorts out the nasties at close range rather than high velocity and foot pounds.
I have used such combinations too many times, with great success when the chips are down and do not see the need to change to lighter bullets.