why would you keep hard booting the phone?? or do you just mean reseting it? once you put the rom on there thats it. its permanent. until you put another rom on there. like putting windows on youre pc. once its there its there until you remove it. yes of course the sony update would still work but there would be no need as youve manually put on the rom yourself! basically at the moment youve got the O2 custom rom on there with the custom O2 menus and rubbish. so you need to find a clean "generic" rom. im assuming youre from the uk from your name so you would need to find the "generic uk rom" the newest version i believe is R3A. have a look in the tutorial section. you just need to find a link to the generic rom, pop it on your memory card and follow the instructions. once youve flashed the phone you can then delete that file off your memory card. remember it will wipe your phone so backup any messages or numbers you want to keep.
you can get what is known as "cooked" roms which have been modified and include many cool extra programs, interfaces etc. have a look at the first post on this website. be warned theres many available! the standard sony roms are also available, i believe the newest is in section 3 of that post. there should be links to everything you need there.
Sony Ericsson Xperia X1 ROM development - xda-developers