I have done the following modification which is satisfactory to me...
All sensor and engine modules give a -10% penalty except survey modules and fleet wide engine boost modules give a -5% penalty. All in addition to their flat bonus as normal.
Ship hulls give..
Transport: Range +20%, Speed -10%, Sensors -10%
Tiny: Speed +20%, Sensors +10%
Small: Speed +15%, Sensors +10%
Medium: Speed +10%, Sensors +15%
Large: Sensors +20%
Huge: Speed -10%, Sensors +20%
Fast and Observant trait gives +5% and +10% in addition to their flat bonuses.
This give some interesting dynamics to the game. It also allow pretty decent long range scanning ships in the later game since you can get range modifiers to offset the penalties during a game. This also make sure you get overall better sensor ranges on your ships the larger the map you play on, but not in the early game when you don't need it.
this... is actually an ingenious way to implement a sensor nerf. creates a nice bell curve:
turn 2 vanilla sensor boat went up to 40 sensor range (higher if you use other bugs)
turn 2 (modded) sensor boat maxed out at 9.6 (not higher with bug)
here's the theoretical math:
3 range per sensor (assumed free tech trait - intuitive, for better sensors turn 1)
2 base range on sensor hull
2 base range from observant
-10% range from transport
+10% range from observant
(4[base] + 0[# of sensor mods](3[sensor mod bonus])) (1-(0[#of sensor mods]/10[modified value to percent]) = 4
(4 + 1(3))(1 - (1/10)) = 6.3
(4 + 2(3))(1 - (2/10)) = 8
(4 + 3(3))(1 - (3/10)) = 9.1
(4 + 4(3))(1 - (4/10)) = 9.6
(4 + 5(3))(1 - (5/10)) = 9.5 - bell curve falls after this, leave the highest scan range at 9.6 for a turn 2 built sensor boat. This is also with no movement as adding engines reduces sensor range as well. The obvious perks is negating the early game 40 sensor range, while in the end game adding REAL value to sensor range boost tech and trade goods. fast speed also hurts vision meaning no highly mobile super scouts, your good vision ships are going to be crawling around, keeping the needed vision in line with good strategy.
for those min max folks, here is a late game look:
assume huge/large hull (+20%) observant trait (+10%) max tech (+25% on thalan tree) and 2 resource (+10%) for a base modifier of 1.65 . max tech sensor gives 6 per unit.
(4 +0(6))(1.65 - (0/10)) = 6.6
(4 +1(6))(1.65 - (1/10)) = 15.5
(4 +2(6))(1.65 - (2/10)) = 23.2
(4 +3(6))(1.65 - (3/10)) = 29.7
(4 +4(6))(1.65 - (4/10)) = 35
(4 +5(6))(1.65 - (5/10)) = 39.1
(4 +6(6))(1.65 - (6/10)) = 42
(4 +7(6))(1.65 - (7/10)) = 43.7
(4 +8(6))(1.65 - (8/10)) = 44.2 - max WITHOUT actively trading for sensor goods (I just pulled a random save - has 58 planets 2 resources) theres also the + 3 from survey...but
(4 +9(6))(1.65 - (9/10)) = 43.5 meh I already did all the math lol
I find the endgame a bit low for insane maps, but manageable. nerfing engine speed the same way (if you didn't already) would be advisable. having a 100 move speed ship may have no sight, but endgame that's still a formidable ship that can blindside you from nowhere if your vision is capped at around half that.