From personal experience I'd say that luggage delivery is probably what could hold you up. Immigration might look very crowded because there are only a limited number of "'windows", but they process passengers pretty fast. (And in any case, any time spent waiting at immigration means less time waiting for your luggage. )

The walking time from immigration to the train station is minimal. Everything (immigration - luggage - customs - arrival hall - train station) is maybe 200 metres (and 2 floors) apart.

And unless you have booked a Thalys train from Brussels to Amsterdam, I agree with irishguy28: any limitations on your train ticket are probably the least of their worries right now. They'll be more worried about getting you to Amsterdam at all. So catching a later train shouldn't be too problematic, in case you arrive late.