"Birkin" sounds like some sort of sex toy
I would be interested in how much of a difference the IRS actually makes. Caterham has a IRS version of the car (CSR), yet many say it is not as good (fun) as the deDion (albeit it is the bigger chassis) and Caterham chose the deDion chassis for their fastest model (R500).
I can't say that my car needs or would be any better if it had an IRS set up
That Birkin website you posted doesn't really have a lot of information about the cars.