I'd suggest a blanket (absolute) change rather than a % based change - i.e. the issue I see is the lag between two units moving towards each other and this being picked up. This delay will be affected by the units' speed, rather than the visual range, hence the delay for the unit to show up once it is inside visual/intel range is the same whether you're talking about a unit with poor vision and intel or large intel/vision.
I also think it should cover buildings that give intel/vision, since they're also affected just to a lesser extent, with buildings getting half of the boost (since the building wont be moving, so the maximum potential distance a unit can move inside intel/visual before being spotted is reduced to just 1 unit's speed, instead of the combined speed of 2 units).
T1 tanks move between 3.4-3.7 speed while T1 mobile arti are <=2.9 speed. I'm assuming (without doing detailed testing to confirm) that there might be a lag of 1 second between a unit entering vision/radar and being picked up as such.
That would suggest an increase of around 6 might mean you almost always have visual/intel of a unit that you would expect to at current ranges. However it would also lead to lots of cases where you get visual/intel outside of the current range. I'd therefore go for half of this to strike a balance, i.e. a +3 increase to visual and intel of all mobile units, and then have structures having half this (so +1.5) benefit.
I also don't see a big issue with scouts or radar benefiting from such a blanket increase - in relative terms it'd effectively be a nerf by being an absolute increase to all units and hence meaning those units function slightly better without radar/land scouts than before.