In my 338 AX which is another version of the 338 Lapua Improved, I have found that for my standard throat design, Retumbo offers the best consistancy to top velocity ratio.
As mentioned Rl-25 also works very well as does Ramshot Magnum.
I feel H-1000 is a bit fast burning for this class with at 300 gr SMK anyway. That said, it works very well, you just have to pay attention because you can get into more serious pressure problems if you keep running the powder charge up with H-1000.
This is not as much of a problem with the slower powders.
I do not feel anything slower then Retumbo offers any advantage in this chambering or similiar chamberings from my testing. You can get some really tight ES numbers but powder fouling generally gets much worse and you will not be getting the velocity the round is capable of. Not that that is a nessesity but if your happy with 338 Lapua velocity it would be much easier to just shoot a 338 Lapua then the improved version.
Just my opinion.