All final salary schemes are contracted-out as they are considered to be good schemes. If you are in a final salary scheme, you have to contract out of SERPS. Your employer has no choice but to use the lower rate for NI as there is a legal statute that says this is corect for final salary schemes. If your employer took the higher (contracted in) amount, it would be discovered by HMRC who would, no doubt, end up fining your employer.
If you are in the Local Government Scheme, you should be fine as it is still index-linked final salary scheme. However, the government is trying to change this, so you need to keep an eye on the news and consult your union rep.
You cannot pay higher NI, but you could put extra money in to an AVC. Speak to an IFA if you're worried.