Old post but I just recently learned about this 130 configuration difference, even when order an "S" level, which I think will even show as a "P300" on the VIN badge, the engine will in fact be an inline 6 but lower output. I have heard from Sarek that it can then be tuned to match the P400 output. So, a fairly loaded 130 P300 can be ordered for "just" $87k usd which almost sounds like a heck of a deal.
The main difference I find the current 2026 choices is that in order to get the slightly more functional seats, the headrest mainly, one must first choose a Defender "X" level trim.

However, it sure seems like one could special order the front headrest parts and pop them in because I would be surprised if the seat structure internally is any different from the "nicer" S trim with "4 way" headrests is any different from the also 4 way but with "winged headrests". The captain chairs might be different but I would also not be surprised if the headrests are equally interchangeable. Other than aesthetics of the full wrap around color of the S tan, vs insert style tan in the X, maybe just the leather itself is different I suppose.