So, here is the answer from Prober about using 2 GB in Pegasos:

1. The stability went down a lot,
2. There were random crashes while using the system for a long time.

So technically it works but there is some problem with memory access. But I haven't noticed this with MorphOS so maybe it's just an OS4 bug.

Prober hasn't his Pegasos anymore (he gave it as a gift to active OS4 user) so he can't check it anymore.

Thank you, Prober, for fast answer :)