Both my DC love GWC. They are now in P2 and P5 and we couldn't be happier with the school. It is big, no escaping that, but the lower primary (p1-3 + nursery) are in their own building and it feels like a mini school, although they are very good at integrating with the upper primary (p4-p7) and then later on with the senior school. The staff we have come across have been fab, and the junior school headmaster George Salmond is a lovey guy, very, very invested in the kids and the school, mucks in and is very approachable.
Assessment wise, for my 2 going into P1....they spent 20 mins with a lower primary teacher, doing puzzles and just chatting. There is no written or obvious numerical testing, although I'm sure the puzzles and chat are sub-consciously seeing how capable your child is. Its very relaxed, they all wait in the hall before being called, there is juice and biscuits, and books/games to keep them amused. Its very friendly, and neither of my DC were fazed at all.
I presume the same is done for the GWC nursery kids, but obviously, they are being constantly monitored by the nursery staff all year, plus the kids are a lot more relaxed as they are familiar with the building and will recognise faces.
We kept my youngest at a private nursery rather than going to GWC, simply because they were happy there, and also we got the benefit of year round care, 8am - 6pm whereas the GWC nursery follows the school term and times, so no holiday cover.
Good luck! have to say, we also visited Heriots and were equally impressed. we just got a better overall 'feel' for GWC, and logistically it worked better for us.