Dean Close has a mixture of boarders and day pupils. Various children I know have gone there when failed to get into local grammar schools and do very well at GCSE, so obviously they are good at getting good results (make of that what you will
It has a Christian ethos, and a new Head just recently.
Cheltenham is pretty affluent. We have Dean Close, Cheltenham College plus obviously Cheltenham Ladies College (which is in a bit of different league regarding intake to the others) so the town is used to the private school brigade. Pate's Grammar School is a super selective grammar school with quite a big intake from the private sector (bit of a bone of contention from us state schoolers!), and gets better results than Dean Close/Chelt College. There was a bit in the Telegraph last year about Pate's v Cheltenham College in their 'town v gown' section, might be worth seeing if you can find that. There are also two girls and two boys grammar schools in Gloucester, plus two v g comprehensives in Cheltenham.
Not sure how Dean Close compares to Cheltenham College academically, much the same I would imagine. Think you might be better posting on the independent sector for the sort of information you want regarding Dean Close, we all tend to be a bit rabidly pro state schools on this section.