I don't see how you get to 20%
outside IR35 contractors pay 19% corporation tax and 7.5% dividend tax = 26.5%
Inside IR35 20% income tax 12% NI = 32% (Because the salary is a deductible expense for the company)
so far only 5.5% worse off.
Now there is the matter of 13.5% Employer contributions but if the responsibility movs to the employer and they determine that the contractor is inside then they can pay the Employer NI themselves just like they do for all there other employees.
Don't panic, Rates will just go up, same as they do now for inside working.