My guess is there is an error in the calculator that isn't taking into account you only having 2 years left in which to make these 'avc's'?
I'd be surprised if the figures output aren't as a result of paying an 'avc' throughout your service and they should be 'factored' for the percentage of your service you have actually contributed for i.e 2/22
I added money to my AFPS75 pension via AVC and there were detailed tables I had to use to work out the actual 'return' I was getting on my investment. As I was on PAS to 55, I expected to easily recoup my investment over the remaining 15 or so years of contributions. However, SDSR redundancy changed that. I suspect I might possibly get back what I invested if I live long enough. I'd be wary in your case of signing up until holding a written confirmation of pension benefit from the additional contribution. You could always start a SIPP and plough some into that (equal to your 40% tax payment)?