As far as I know a Xenon filled bulb will not give the same brightness as a full on HID kit.
The kits may be E approved,but there is a grey area here that I am not educated enough to comment one,all I do know that if your ever blinded by a HID equipped car it can be bl00dy dangerous to the poor sod coming the other way,and thats with all the equipment that manufacturers fit as standard,with an aftermarket kit it can be even worse.
I must say here and now I am not a fan of HID lights as they don't really give any benefit to anyone other to those people that have them,and have the opposite effect to other road users.With a decent set of projector dipped beams you can see all you need to,HID's are just another way to sell cars and to convince people that they must have them.
Thats just my opinion though,others might disagree.HID's are usually only fitted to dipped beams,and in that case they are pretty useless.You can fit what you like to main beams and as long as you dip them at the appropriate times then thats fine by me,dipped beam is another case.
Front foglights were added to cars to 'aid safety' but are only used in the correct place and time in 1% of cases,with HID dipped lights they are used all the time no matter when they are appropriate,if there is ever an approriate time.It's just another way to sell cars and for people to show off and say they've got 'Xenons'.