This is going to be blunt - sorry :-)
8-10% is not fair and not a good deal. Because you've already given the CTO a different figure that they have been working under for the last four months.
You change things from under them now then you're:
a) Morally in the wrong as far as I'm concerned
b) (Obligatory not a lawyer warning - go ask a real lawyer) probably legally in the wrong depending on your location. Verbal contracts and all that.
c) Got a pissed off CTO - which is going to effect his work, and the work of the people they manage.
d) What if the CTO walks? How much of a hit is that going to put on your ongoing progress? (If somebody did this to me now I'd just walk straight away. If somebody did this to me in my 20's I'd probably grit my teeth, grin and look disappointed but chipper, and immediately start looking for a new position. The message it sends to me is "they broke their word - I can't work with people I don't trust".)
e) What if it walks and writes a ranty blog post that gets picked up and goes viral? How much PR damage can "screwing over employee #4" do to you if it goes public?
f) How much harder is recruitment going to get once it's know that employee #4 was screwed over? (and it will get out - even if nothing goes public. People talk to each other. I have a mental list of companies I don't do business with and don't recommend because of similar stories that have never made the headlines).
At the moment, if I were in your shoes, I'd do the research for folk in that area and try to negotiate around the lower-end of the range that you initially gave. I really would not try and drop it beyond that. The risk is way to high.