The US east of the Mississippi isn't appreciably less densely populated than Europe, and given the general age of the eastern track routes compared to Europe and the topography (Appalachian mountains need a decent amount of tunneling) and likelihood of overpasses (the road network is probably as dense in the US as in Europe), I can't imagine that there is substantially more work needed to adjust the loading gauge in the US for double-stack than it would be for Europe.

Probably the big differences are that a) the US already had a comparatively roomy loading gauge compared to Europe before conversion was necessary and b) the West Coast-to-Midwest routes were probably more likely to be double-stack-compatible before the projects started and provided a seed route to develop the concept before the expensive loading gauge conversions in the east had to take place.

Electrification doesn't affect double-stacked routes other than needing (I think) an extra ~2 ft of height compared to non-electrified track. The cost of relaying catenary cables isn't going to be all that high: electrifying track from scratch in the first place appears to cost about $1 million per km, and a lot of that cost will include the substations which don't need to be replaced; actual costs of replacing the gantries is likely to be less than half that. That's of course assuming that there is actually room above the track to support the new height.