more hp won't allow a higher top speed in any gear, unless you are in your top gear, and were drag limited (the previous hp that you had could not over come the windresistance, hence you topped out due to that) but it will allow you to accelerate to that top speed in whatever gear faster.
do raise a mph in a gear, the actual trans gearing has to be taller, or the final drive ratio would have to be lowered, or larger tires and rims could be added to do the same, you'd just have to make sure the overall ratio made the tire taller.
of course with any of this, you will probably have an inaccurate speedometer.
the other way to go about it would be to raise the rev limiter on the vehicle. here is a javascript program to do your own calculations if you were to alter any of the parameters i mentioned above.
you need to know your tire sizes, rim size, gear ratio and front diff ratio, let me know if you have any other questions
http://www.sgmotorsports.com/mph.htm#GearSpeed