Higher bhp cars normally have shorter intervals but if up to 450bhp 6k intervals should be ok . A good idea is to change the oil and filter between for more protection.
Don't take my opinion though as I'm basically a mechanical incompetant - speak to Abbey MS/GT-ART/Rod Bell and I'm sure they'll give you prices and intervals. See the links page on www.gtr.co.uk for their contact details.
PS can't help on costs as I keep having extra toys fitted everytime mine goes to Abbey so don't know the price of a standard service