The .35 Remington in an XP-100 will easily handle 200 yard shots. The action is much stronger than the Marlin Rifles, and can withstand loadings substantially greater than those ordinarily published. Pick up an old copy of "Handguns and handgun Hunting" (I think that is the correct title) by Larry Kelly and J.D. Jones and you will see what I mean.
I have a .358 Winchester in an RPM XL Hunter with a 14" barrel, and it will only best my top end XP-100 .35 Rem. loads by 10-15%. [The XP-100 loads in question are with H-322 and AA-2520 and a 200 gr. bullet. I will not post them, as you need to work up loads in your own gun.]
I discussed re-chambering my XP-100 to .358 Win. with JDJ, and he did not think it would be worth that difference over what I am already getting with the .35 Rem. Admittedly, I have not spent the time pushing the limits of the .358 Win. RPM, and it would probably do better.
If you want more power, you can convert to .350 Rem. Mag., but a fiberglass reinforced stock, and a brake/ported barrel are a MUST. An easier conversion would be to 35/284 Win., which is close to .350 Rem case capacity. This is a handloading only proposition, but that is pretty much true of the .358 Win. these days.
All that being said, IMHO in the interest of cost and user friendliness, I would leave the XP-100 as is. The R-P 200 gr RNCL will do a good job with the heavier loads, as will the Speer 220 gr FN. The Hornady 200 gr Spire Point has a more aerodynamic profile, but I find the jacket material to be somewhat tougher than with their 200 gr RN. It is hard to beat AA-2520 for accuracy, uniformity, and reasonable pressures with heavy (200+) bullets in this cartridge.
Good Luck. Sawfish