Should not result in any improvement, A 390 is just a 290 bundled with 8GB of RAM.
Not quite - the ram was also faster (1500 vs 1250 clock).
I think the R9 390 also implimented the "boost clock" stuff in BIOS but not sure on that.
I'm not sure if the R9 390 added PCI-E 3.0 compatability.