Re:The customer insisting you fit them; you are the professional who knows the implications of doing so and what's right, they are not designed to be fitted into normal light fittings (manufacturers instructions take precedence) so I'd educate him of that fact or walk away. Drawing 450W per light (how many of these are there in total) and the other lights, is the cabling up to the job?
Re LED costs. If he wants to consider going to LEDs, that would save about 15% off his electricity bill and if you'd like to put him in touch with me, I can arrange for good quality ones to be supplied and fitted for free throughout the whole house (and no, it won't be me doing it or getting paid for fitting them, and if you'd like to PM me for more information, you could get paid up to ÂŁ350 for the introduction).