It's fine Baz. 90 degree bend proportionally increases restriction on total throughput through loop. (I can say I've had more experience than lot of people here in water cooling... what can you do when phase change was not around? :P)
The real problem as Pete has mentioned is that since all water blocks are very restrictive compared to regular plumbing, the impact shows greatly. In this case though, it isn't too much to talk about since loop is very short as well as having not much of restriction. Of course, long radius bend always is better but if space doesn't permit, why bother
Looking great man