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. . would the formula for a 300 grain JHP work for a 300 grain XTP and vice versa?
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This is a yes and no kind of thing. Yes, when you are talking about "starting level loads" all bullet of the same weight will work well enough with the low level loads.
BUT, when you move past those, then you get into a range where the bearing surface of the bullet (the part in contact with the barrel) AND the composition of the jacket material (some are softer than others) can and do have significant effects.
A lead bullet and a jacketed bullet can weigh the same but each has a different resistance to the rifling digging in as it goes down the barrel. Two different jacketed bullets can differ in their resistance as well, a long bearing surface has more resistance than a short one, a harder alloy jacket has more resistance than a softer one.
All these things play a part in how soon a load reaches the max allowed working pressure, and what velocity is obtained at that pressure.
To further complicate matters every gun and ammo is slightly different. Sometimes a listed max load isn't the maximum safe load in every gun, but was in their test gun.
Most of the time, most combinations fall in the middle of the bell curve, and this is why reloading data is useful as GUIDELINES. But one needs to be aware that their test gun and ammo components are NOT identical to your gun and components. No matter how closely you try to match them, they will not be identical. GUIDELINES, not laws.
Everything made has tolerances. When things line up as usual you're in the middle of the curve, but it is always possible things will line up so your results are at either end of the curve. And there is NO WAY to know until you pull the trigger. If you're on the low end, you don't get the performance you want but otherwise, you're fine.
BUT, if you're on the high end, you might be at dangerous level just starting out, which is why we always say "start low, work up, slowly!"
Every combination of components and firearms can be different. Sometimes radically different. This is why everyone's data is different.
again, GUIDELINES, not rules or laws...
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Maybe I am missing something here, but why not use Hornady data for loading the 300 XTP?
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I would use Hornady data for a Hornady bullet. They test them. Understanding that the results I get should be about what Hornady got, but COULD be different, and start at the low end to see what actually happens in my gun in my hands.