You are a runner. Now you are cycling.... start swimming and join us on the Tri Side!!
So I have recently been riding my 10,2kg steel bike as well as my 7.3kg blinged out Venge with Dura Ace Di2, deep section wheels, the works. I've actually had rides where my average speeds on the two bikes were exactly the same for the same power, although that was admittedly a bit misleading, as winds were different. That said, I will estimate about 1-1.5kph difference in going to a lighter bike.
I will say this, though - on the road, a lighter bike just FEELS a lot more fun to ride. Can you get faster on your current bike? Absolutely. But if i were in your shoes and had the funds, I would TOTALLY spring on a new, lighter carbon bike.
Personally, I would NOT spend the money in lightening a 25lb bike. I mean, how low can you get it? 22lb? That's still a pretty heavy bike. Might as well use it as your exploring/off-roading/rain bike and put those funds towards a lighter bike.