I can’t speak to use on game, but I have done a lot of research on these cartridges recently and have about 15 years of experience with reloading and long range shooting as well as hunting in general (just for background).
The 458 Lott as you know was invented to solved the issue with the 458 win mag needing compressed loads due to the powders available at the time not having enough bulk density. This let to clumping of the powder which is no longer an issue today through powder and coating enhancements as well as new higher density powders like re10x and 2230. This was all with a 500 grain bullet. So in short the Lott was optimized for use with 500 grain lead jacketed bullets with less compressed loads to achieve the same or slightly higher velocities as the 458 win mag.
Neither of those issues exist today with modern powders and the knowledge we have with reloading practices. Today, the Lott is able to produce 100 fps more quite easily with these modern powders with the same bullet. The key takeaway here is the advantage of the Lott from its original purpose is null and void at this point as the issues with the win mag were address. Sure, you can get more velocity with the Lott today with modern powders but the recoil is significantly more as well when loaded to equal pressures
Another key point is that the freebore on the Lott is actually shorter than the win mag. Therefore a long loaded win mag performs nearly identical to a standard Lott offering. Of course you need a longer Lott length magazine to accomplish this. Basically in a word what I’m trying to say is the Lott is limited by freebore while the win mag is limited by mag length (in rifles with standard length actions and magazines). The win mag is limited by case capacity in the shorter 3.34” standard length action and mags.
Therefore, imo a 550 grain bullet would be too long for Lott as you are still restricted to the freebore of the chamber, thus by loading it shorter you’ll be eating into more case capacity and have lower velocities before pressuring out. On top of that the Lott with a 500 grain bullet already has plenty of stopping power as indicated through anecdotal evidence and also has a lot a recoil. A 550 grain bullet isn’t going to add much additional performance or value, if any at all.
Today’s bullets are far more advanced with solid monometal bullets. A 450 grain bullet has penetration and performance that meets or exceeds the older 500 grain bullets the win mag and Lott originally shot.
My advice is to load either a 450 or 500 grain CEB solid and pair it with a 420 or 470 grain raptor respectively. This will allow you to take any game on the planet. My personal choice would be the 450 and 420, but you certainly can choose the heavier ones with the Lott because you have a longer case and magazine. In my experience the more speed you give a solid, generally the better performance and penetration you’ll get. These bullets will not have deformation issue until 2800+ fps. Both of which you don’t need to worry about with this cartridge with these bullet weights. Max you’d see is maybe 2450-2500 with the lighter bullets I mentioned and around 2300-2400 with the heavier ones hand loaded properly.
Just my .02 if I was in your position. There are other great manufacturers for solids like NF, etc so by no means do you have to use CEB, that’s just what I personally shoes due to availability and previous performance I’ve seen using them.
Good luck