well, there are some things id do before buying new wheel
1. practice practice practice
2. watch faster racer lines, you might be driving a wrong line
3. try to get a better, faster set, it might be that you arent using the best set, so therefore u get slower times
4. try to make a hotlap and then compare your spr with wr's, so you will see what and where are you driving different than wr is.
all of that is easier then just getting the new wheel. and it will most probably make you faster than a wheel would. and as for wheels, im Logitech DF GT user, its a big step over FF EX i had before, gained around 3-4 tenths on my pbs depending on track, so yes, a better wheel can get you better times, but there are other (more important) ways to lower your pb, because good wheel cant make a shit set go faster.