For anyone else facing this issue, this is what I decided to do. The trade-off is between weight, strength and cost. What's right for me may or may not be right for you, but this is what I learned in the process. Hopefully it will save you some time, and if you know or learn something more, please pass it on.
Regarding strength, Gravity Cast wheels are weakest for a given weight, Low Pressure Casting is next, followed by Flow Formed, then Forged. The end result of these different manufacturing processes is a different molecular structure, with Forged producing the most grain-like structure, followed by Flow Formed, while Cast does not produce any type of grain. Grain equals strength.
All of the wheels I found that use the term "Flow Forged" are really "Flow Formed", not "Forged". "Flow Forged" is a marketing term that seems designed to disguise the construction of the wheel. Flow Formed wheels typically use a Low Pressure Casting for the center of the wheel and a rolling process for the rim that helps to create a grain-like structure for the rim. So, for a given weight, they are stronger than Cast but not as strong as Forged. So, just because the term "Forged" appears in the name of the wheel, or the name of the manufacturer, do not assume these wheels are Forged.
There are also differences within each type of construction, so it is hard to make direct comparisons based only on the manufacturing process. For example, Forged wheels can be manufactured with different alloys and at different pressures and temperatures. The higher the pressure, the stronger the wheel for a given weight. Many are formed using 4K - 5K psi and some go as high as 8K to 10K psi. So it is not always possible to draw a direct comparison between Forged wheels from different manufacturers. They are not all created equal.
Regarding cost, Cast are the least expensive, Flow Formed are around 2x the cost of Cast, and Forged are around 3x to 5x the cost of Flow Formed, with the highest pressure Forged wheels typically being the most expensive. In addition, some wheels are also milled before delivery to remove non-structural material and further reduce weight. Milling adds to the cost of any type of wheel.
Regarding weight, it depends on the purpose of the wheel. High performance wheels need to be as light as possible, to reduce unsprung weight. Wheels that prioritize cost over performance will be heavier as they rely on less expensive manufacturing methods. 20" wheels, like those on the RS5, can range from 7.7 - 9 kg for high-end Forged wheels, to 11 - 12.7 kg for a Flow Formed wheel, to about 18 kg for Gravity Cast wheels. Is an 8 kg Forged wheel stronger than a n 18 kg Gravity Cast wheel? I don't know and no one seems to publish any specs that would answer that question. Is an 8 kg Forged wheel stronger than an 8 kg Cast wheel? Absolutely!
So, the right wheels for you depend on why you want the wheels, and your budget. I wanted strength to survive bad roads, but also wanted the lightest wheels that would provide sufficient strength to get the most from the RS5. BBS seems to be one of the best manufacturers out there and they invented Flow Forming. 20" Forged wheels from BBS weighed about 9 kg and typically cost around 8,200USD / 7,200EUR / 6,300GBP in our area for a set of 4. Flow Formed wheels from BBS weigh about 12.5 kg and cost around 2,400USD / 2,100EUR / 1,850GBP. Cast wheels are everywhere, they weight a lot (or they are not strong) and cost around 1,000USD / 882EUR / 772GBP for a set.
For me, the right balance came from Audi. My RS5 came with milled forged wheels that weigh about 12 kg, so they weigh more than BBS forged, which probably means they have more strength. They weigh about the same as BBS Flow Formed wheels which means they are stronger (because they are forged). So, they seemed like the right compromise for me between weight and strength. I would have purchased the BBS wheels if I could have found any indication they were as strong or stronger than the Audi wheels, but I could not find anything to that effect. There is lots of glowing marketing language around strength, but nothing concrete. Finally, concerning cost, I was able to negotiate a price break on the wheels from my Audi dealer, given I just purchased the car. So, I decided on milled forged wheels from Audi that provide the same weight reduction as BBS Flow Formed, are probably stronger than BBS Forged wheels I would have purchased if I had more concrete information, and cost less than the BBS forged wheels thanks to my dealer.
I spent a lot of time on this. After all, it's winter, I just got a new car, and this was more fun than all the other things I should have been doing. While there are lots of opinions out there, the published facts are almost non-existent. So, if you have anything to add to this, please do.
