Hey Thom,
leaving all the hardware behind and working only with a laptop is exactly what I’ve been doing since Cantabile 3 came out. It’s been an intensive process and quite a bit of work, but to me totally worth it.
For me, the sound I’m getting with my laptop setup is actually better than the stuff I’ve been getting from my workstations I used to carry around - I have great VST instruments that specialize on specific instruments like piano, hammond or analog synth sounds - these actually beat most of the sounds that I could get out of my hardware compromises (not everyone gets a grand piano or a real Hammond or a 10-keyboard monster setup carried around by roadies…).
The key point in this setup is setting priorities: what are the key sounds that need to be as good as it gets? For these, you need to really invest in the right VST instruments - and invest the time to tweak them; same as for any hardware synth. For the rest, I’d recommend a combination of the VST version of the Korg M1 and Xpand!2 - both are good “bread-and-butter” choices.
You’ll find long threads on this forum on the merits of one or the other organ, piano or e-piano plugin - I won’t duplicate what they say here, but there are really good options in this field. Same for any virtual analog synths - broad offerings of really professional-sounding stuff available.
The only thing that I would consider keeping outside the laptop would be voice processing. For me, the round-trip-latency involved in getting the signal into the box and back out is too distracting as a singer (funny enough, as a guitarist, I can live with it for my guitar amp plugins). And if you use any voice processing effects like the VoiceLive series by TC Helicon line offer (harmony etc), you won’t currently find a reasonable software alternative for live use - trust me, I’ve searched high and low…
So for the moment, my minimal setup contains a laptop, audio/midi interface and a master keyboard plus a VoiceLive Touch for vocals. I’ll even leave the master keyboard at home and bring only a controller box (Novation LaunchControl), some pedals and a MidiExpression when there’s a trust-worthy keyboard on-site.
On leaving the hardware at home: I’ve personally taken the plunge - I don’t even wire up the audio out of my Kurzweil PC3 (which I use as a master keyboard) - but I always have a backup laptop setup (complete with audio interface) with me - easy to carry and can be a life-saver if something goes seriously wrong with your main rig. Try this with a classic hardware setup 
…
Cheers,
Torsten