You are correct, mostly, all things considered can make a big difference in the outcome.
If the tires are the same weight and the wheels then the weight is still the same as well as still pushed out pretty far from the center so there might be a little gain or even loss....depending on exactly where the weight is located.
The BBS are 17x7.5 which I wanted a wider wheel, +35mm I can work with but would of been better in +40 or so but they weigh around 4lbs more each, ouch!
The Konig Feather in 16x7 is a little lighter than the 17x7 but the PS3 AS is heavier in 16 than 17 sizes I would use so there is a bit of weight gain and more cost so not a great way to go and I have the W rated version but would have to get the V rated to get 16's
I want to run 15x9, 12lb wheels with 21lb or so 225 wide tires for the street, nice weight savings and would look great but no tires made I want to use as need an ultra high performance all season.
I could run the RPF1 in 16x8 with the PS3 AS but still not much weight savings for the added cost, might not clear my BBK due to the wheel design and I have had many sets of them so want something a bit different. The wheels are 2 lbs lighter than the 17x7 Feathers but the tires are 1lb heavier so saving 1 lb per corner is not a huge improvement for having to spend at least $1,000 more than what I have already spent.
16" wheels are tires are falling off the radar pretty quickly as well as great all around street tires in 15" wider than a 205 leaving 17 the best choice in many cases.
Good news for 4x100 converted cars is the new MX-5 is back to 4x100, I do not know the offset yet but have talked to the guys at Godwinracing and 949, there will be new 17's in flow/roll formed wheels for great prices coming out, just not right away.
I have looked into having some 4x108 wheels made but the number of wheels needed to order is just to large and quite a delay to do so. Perhaps one of the vendors that is not retired and just having some fun like I am will pick up the idea and make it happen for you guys, I truly hope so.