The stock 17" alloy on the 2018 SL weights 47.2 lbs mounted with brand new 215/50 17" 95XL Bridgestone Ecopia 422 Plus. I have a set in my shop waiting to go on the car so I checked one with a digital scale. They are noticeably light for their size relative to our other cars.
These tires weigh 21.4 lbs, so my rim should be in the 25.8 lb range as Herding has posted. TPMS and balancing weights likely make up the difference.
Any reduction in rotating mass will make a difference on acceleration (like a lightened flywheel) but keep in mind that your tires will still be the same mass, and the effective gain found by decreasing rotation mass will decrease as you get closer to the hub. If you visualise where the mass is distributed in the rim vs the energy required to accelerate the mass, it makes sense. You should see at least a small improvement in acceleration, and if your driving is urban, you may also see a small increase in efficiency. On the other side, regeneration may actually be reduced a bit as it will be a bit easier to overcome the lighter rim/tire's rotational inertia.
You should do a few rolling tests (same stretch of road) and time 30-60 runs before and after...post up your results
On my autocross car, I can very much measure the difference on different rims/tires with respect to acceleration, particularly when the rim is a different size. This article measures an 8lb or so weight saving per wheel as a an increase of 0.4 seconds or so on the 0-60 times with a Miata (much lighter car): https://grassrootsmotorsports.com/artic ... ly-better/
The article also makes a very good point on how reducing rotating mass vs just making a car lighter to the car is quite different. Their results are consistent with my own observations, particularly "in gear" acceleration on the highway with a lighter rim/tire combination where grip and wheel spin are variables you can mitigate (vs the 0-60 runs). That's why I'm suggesting you just measure 30-60 times.