My cousin also installed the 50W-bulbs yesterday. He put a post in a Volvo-forum, similar to this, and got a reply from a guy who has used the 50W-bulbs for half a year without any problem.
David C Klasse
I don't know what wattages that is available. There was a website-address in the package of the bulbs. Maybe you could try their homepage to find out what wattages they produce. When thinking of it the bulb is just a standard H(x)-bulb and the acctual "invention" is like a converting sockel making the H(x) bulb fit in the position for the back-up-bulb so I guess that means you could buy any aftermarked H-bulb to put in the sockel.