Wildcat,
I dont think you can acheive that kind of speed from that case without ruining them. 25grn drop in bullet weight will not add 300fps. Maybe 100fps 3100 maybe a ballpark on a 275grn bullet. A standard 338 Lapua with 300MK runs 2900fps tops, improving it will get you to 3000fps in my estimate. The case capacity is just not there, a 338-378WBYimp or 338-416Rigby-imp will get what your looking for. Sure you can push cases to past the limit and get high velocities, but at what cost? In all instances find the load that shoots the most accurate and keeps the cases in tact. Kirby and I can push the big 338's we have to past 3500fps with the 300grn pills, but we will loose the cases, primer pockets will start to open up, maybe two firings? Brass starts to extrude just past 65,000psi, so if your looking for 3100 out of your Lapua Imp, you will be in that range. I'm not saying it wount work, just becarful as your at the top of the spectrum.
Good luck,
Dave