When user hover over the searchlight turret, show a radius shape of upgrading turrets (not yellow, use some different color)
The same if user has the searchlight turret in the hands (building mode, or if a ghost – planning mode) — follow cursor position.

Great minds think alike, ray1claw had a similar suggestion! https://mods.factorio.com/mod/SearchlightAssault/discussion/624188b0743ae7630edbeb2d

I was considering adding a terrain decoration (wires poking through the ground?) to show the upgrade-area.

Not sure when I'll have time to work on this, but it's a good idea I want to do some time.

I do not like idea with decorations — many users do not like them, and install some mods with a tool to remove decorations.
The better idea — use an overlay with zone, like Factorio already has — Roboports, Electric poles, and etc.

at first — The overlay — it is a consistent look with other parts of the Factorio game.
also, it should be temporary — the overlay should be visible only if we build something, or on hover, and not distract at other times. Like you already do for searchlight's active zone.

I've just released version 2.2.4, which adds some visuals to help make boosted turrets more distinct, and displays a radius for boosting turrets when holding / hovering over a searchlight. The game engine doesn't make it easy to control what color the visualization appears as, so I had to try making the overlay distinct from the electric zone.

The new version should start to appear in the mod portal shortly.
If you don't see it in-game you can download it manually here: https://mods.factorio.com/mod/SearchlightAssault/downloads