It seems to me that your own question is contradicting itself.
Namely, you say:
Adding Paypal would replace our card details and address boxes with a link through to PayPal offering them a simpler (and faster) way to pay.
But then you ask:
Are there any studies which show adding another payment processor (Such a Paypal) increases UX by speeding up the process in a way that they are more inclined to complete the purchase?
Basically, adding, yes, if done properly. Replacing, I'd wager not just no… Hell no!
PayPal is used quite widely, but there's a very vocal set of anti-PayPal users around, alongside a set of users who cannot be bothered to (or don't know how to) sign up with PayPal.
Keep these additional consideration in mind:
Firstly, or so it did last I checked, PayPal only ever allows you to use a credit card once without creating an actual account. Because, it gets tied to an account in the background, but that account is not associated to an email — i.e. you cannot get the password reset. Speaking personally, I got burnt by this at least twice when trying to make a purchase, and I lost a few customers myself over the years due to PayPal disallowing them to re-use their card. (Things might have changed since, but I've very little faith that it did.)
Secondly, letting PayPal manage your checkout/authorize/capture/chargeback/refund process is a mess of biblical proportions, unless you're in the business of shipping physical things using FedEx. Anything short of that is subjected to random capriciousness, and not just by customers; also by PayPal itself. Your funds get randomly locked for weeks on end because your business is unusually active over the WE, customers receive your product and get a non-appealable refund directly from PayPal, you name it. It's ugly, the seller is always on the receiving end, and getting someone from PayPal on the phone is about as easy as getting the President of the USA.