I really don't think it's as simple as slapping a weight penalty on cars to slow them down and vice versa to speed them up. If the system was easy then iRacing would likely have made it happen as they know a lot more about the system than all of our combination of input here.
I did find something interesting on Reddit though in regards to BOP at Interlagos and why the BMW was quicker than other cars that week. Apparently it's due to a combination of altitude and the BMW running more Turbo Boost than the other cars it was compared to, in this case the McLaren and Ferrari.
Legend:
BMW, McLaren,
Ferrari
My guess is it's very, very complicated and probably based on everything if the above graphic is any indication. For all we know BOP takes into account temperature, humidity, altitude, track state, road compound, tire wear and heat distribution, time of day, rear and side view mirror adjustments, empty candy wrappers in the glove box, whether or not you shaved, had a big lunch or just took a p00p, etc.
Lastly, I think higher iR Drivers or wannabe's tend to focus on this stuff more than the bulk of iRacing's Membership.
I've been watching this guy on Twitch called Piltdownman (I think it is) and he's approx. 1.2 K iRating and there's a pretty good distribution of manufacturers in his splits.
I do agree it sux to see all one manufacturer in a race and wish it were different but iRacing's system allows for it. I'd much rather see series where you're locked into a manufacturer for the entire season but I'm very sure I'm a minority in this regard. Plus, iRacing simply cannot force anything as it's all about options, remove them and people will play something else.