I suggest you read all the "stickies" - the topics at the top of the list in this Bucks section. They will give you details on your initial questions and others. Meanwhile, some short answers:
Is it the same test used by all Bucks grammars?
Are there enough grammar places for the top 25% of the population (if they all toook the test and were in the top 25%)
I believe so.
Do all schools just take a basic pass or do some select on score in the test? (superselectives)?
Pupils only have to get the qualifying mark. There is no advantage in getting a high score (other than pride, perhaps). If there are more who qualify, live in catchment and put a school first choice, then after special needs and siblings, distance is the deciding factor. There are slight differences between schools about the tiebreakers, but for all of them, you only need a qualifying score. The Bucks County Council website has a downloadable admissions booklet which gives all the details.
If there are superselectives, what is the general feeling in Bucks towards them - would it be preferable for them to select in the same way as other grammars, or is the "choice" it offers good?
There aren't any (see above). There are some grammars that are perceived as having a little more prestige than others, but in terms of results, they are all fairly alike. However, there are BIG differences between the best and worst of the upper schools (secondary moderns).