I edit on a pretty powerful desktop with a calibrated, wide gamut monitor, profiled printer/paper/ink and so forth. I've not found anything reasonably comparable and reasonably portable, my sometimes limited imagination prevents me from even considering any comprehensive post processing/editing in the field.
I suppose if I was heading off on a Safari, I'd find an acceptable compromise solution.
Meanwhile, I'm quite satisfied, delighted actually, tethering with a (far less expensive than an SP3) 10" ASUS Android tablet using DSLR Controller
Chainfire, the DSLR controller developer has released an Alpha build of DSLR Controller AFMA
though I've not yet tried it.
Same dev has a WiFi adapter gizmo if you'd like to go that route, some fabrication and assembly required.
There's also Helicon Remote
for Android with similar functionality from the same folks that offer Helicon Focus for PC and MAC, I just now see they also offer Helicon Remote for PC and MAC as well as Android.
I'm delighted enough with DSLR Controller that I've not been tempted by Helicon Remote though I'm quite pleased with Helicon Focus for Macro Focus stacking.
edit....... One quirk, maybe it's a 'feature' I've found with DSLR Controller, the program displays an error on connection if you've set to Back Button Focus.
Since so many of my starting settings when tethered differ from my handheld preferences, I've set one of my custom shooting mode settings to full manual, live view with exposure simulation, shutter button controlled focus, works a treat.