Sorry for my mechanical ignorance. What do you mean when you say too much concavity?
The wheel manufacturer has this model for the Lotus Emira.
I think they mean that the final wheel will not look like this, they will have a flatter spoke profile and not a concave spoke profile like the rendering on the screenshot.
To fit 9” and 11” wheels under the car the offsets will have to be in the high 50s.