Hi Drazen,
I use Aja LHe this way and the quality of downconversion is great. I do audio post, and can monitor HDV or ProRes on full HD lcd monitor(with Aja Hi5 or HDP converters), and simultaneously SD on analog Sony monitor. I alway prefer analog monitor for audio sync check and analog color looks much better to me too.
Before you jump into ioHD I can show you how it looks on LHe, we live in the same city. I also have an ioHD on order, but Aja just moved the delivery date for another 4-6 weeks, on top of 24th September that was announced on IBC.
regards,
Peter