Hey dude, I had a 401 for the longest time and upgraded to a JVM as it had the same kind of sound(which i loved), but kinda did more. The 215c's the 2 channel one yeah? They'll be really similar in both volume and tone, but the 401 is simpler so the sound is kinda truer.
Is it a real improvment ? Not as much as you might want
Is it still usuable at home (50W instead of 40W but still with the same valves, I think) ? yeah
Is the Overdrive section more pushing than the DSL401 ? a bit, not massive, more synthetic
Are the programmable footswitches a good point ? yeah, but not too much over the 401's
Which one is better : the JVM 215C or the 205C (with 2 HPs) still at home level ? Don't you need to push more the volume to make them move ? 205 will be MUCH louder and will not be great for home use
To be honest i would buy a second hand 2x12 cab whatever you do, as this will be plenty enough for ANY gig, with the option to not use it at home.
I went from 40w to 100w so that was a bigger difference, but I genuinly miss the drive ability of my 401, it's such a versatile little amp.
As far as metal riffage goes, just get either a MXR dist pedal (very organic still, but damn mental) or use your yellow channel in conjunction with a tubescreamer, and that will give you loads as long as you keep your amp gain about half to prevent buzzy overgain.