if you have 10K users - your 2K concurrent users doesn't sound quite right - what do you really mean by "concurrent"?
10K users on a Moodle site pointing at an MS SQL database
"might" work ok - but to support 2K users all doing a timed quiz at the same time is going to be a signficant challenge and would likely be very costly compared to similar hardware running mysql
The biggest issues I've had with performance tuning a site running MS SQL as the back-end database has been around the PHP drivers for MS SQL - they just don't scale in the same way as Postgres/Mysql - this is a limitation of the PHP drivers built for MS SQL rather than Moodle or whatever server power you poke at it.
And then... you have the problem that the big sites (at a much higher scale than you're working with here) - all invest time and effort to make sure that the performance of Moodle's internal queries run really well on mysql/postgres based databases, and there are a really small number of competent developers running a Moodle system with an MS SQL back-end.
I assume "cost" is a factor here - in which case it will be cheaper on server hardware/licensing to use mysql or postgres.
But - as Visvanath mentions - before you go too much further - make sure you read the similar posts here in the forums - and then come back with more specific questions.