If these applications are outside your usage scenario, buying the 295 model is basically a waste of money. The 290 model is absolutely OK for the usual desktop needs.
When the fan is below 1200 rpm, which is most of the time when you're not solving [strike]massive systems of linear equations[/strike] playing games, my imac (i5/m290x) is mostly silent. If it's much above 1500 rpm, it's audible. 1750 rpm, it's loud.
The Heaven benchmark, with the music off, is pretty loud.
(Your office environment and ears may vary)
The "horrible UI lag", when it appears, is associated with transparency effects, and those can be toggled through the Accessibility control panel.
I'm not sure if the m295 is happy with the fan at 1200 rpm. It may well be.