I've not been able to found numbers on a ratio of weight vs acceleration difference.
According to this World of tanks forum post (last answer), weight affects your acceleration and hill climbing ability.
Top speed, acceleration, and turn rate. Top speed appears to be fixed per vehicle. Acceleration (and hill climbing speed) are based on power/weight ratio. More powerful engines increase acceleration and hill climbing, more weight (from modules or equipment) decreases it.
Tracks change your turn rate and up your weight capacity. Grousers, torsion bars, etc bump your wight capacity. The more of that extra weight capacity you use, the slower you will accelerate or climb hills. Better drivers/commanders/vents bring all 3 closer to their stated 'perfect' stat.
Also stated in this thread, first answer
a) Weight does affect acceleration but top speed is fixed. Weight limit has no effect on acceleration.
Also further in that thread :
And yes, a tank without equipment will accelerate faster. Although a one ton spall liner makes a much bigger difference on a 15 ton tank than it does on one that's 100 tons. I've found about 5% extra weight is noticeable and 10% and easily noticeable so most of the time equipment doesn't make much difference.
So for your case with the Luch, you should notice a difference in acceleration with the lighter gun since the tank is a light one.
Also, note that the wiki states
Once upgraded, the Luchs gains incredibly high acceleration, the ability to turn on a dime often causing it drift when turning at high speeds.
With the best engine, you might still notice the difference but the pros of having a upgraded gun might be worth more than the increase in acceleration since the top speed will remain the same. Rare are the maps where you should not be able to reach near top speed before encountering enemies.
You'll need to try what fits your playstyle best but for the information available, if you have the best engine, you should get the bigger gun and use half a load of ammo to reduce some weight, remove any equipment that are not needed for your scouting/playstyle and still be able to dish some damage while scouting efficiently.