yeah FTE will do 250 rwhp without breaking a sweat - generally considered limit here is 400 rwhp and 750 ft-lb or a bit more, that’s with a stock fuel pump and untouched internals. But upgraded injectors (+70s), bad boy Gturbo, DTE chip, 3” exhaust, upgraded FMIC, upgraded air box. You then need a very special clutch - NPC do a 1300Nm one but people have problems with them slipping too.
I reckon for 250 you’d need at least another turbo and chip, obviously with exhaust and probably intake improvements. Mine’s running a stock tune and turbo with 15.9psi boost, 600x300x76mm FMIC with 2.5” intake pipes, HF Safari snorkel, 3” exhaust, blocked EGR and cleaned intake, no intake heater and still pulls 30” H2O at the filter. Generally accepted vacuum limit for all diesels is 25” - I hate to think what it would pull with a chip, and the 80 box and filter are even smaller! Yeah an FT should do it ok too - probably exhaust, turbo, intake mods maybe a FMIC, possibly injectors.
You’re right FTE has a few internal improvements over FT, nothing particularly significant though. The pump is the main and massive difference. Very common swap here into 80s, wiring isn’t hard or complicated if you’re prepared to get your head round it. If you pull the FT out send it to Australia- they’re going for about AU$14k currently, a lot of the time more than FTE with looms/ecu. I’ve noticed quite a few people here importing FTEs from UK, looking at 100 prices their about 1/2 to 1/3 of aus prices and much lower km.