Nope, will have no effect. Spring rate is spring rate.

Run the shortest spring you can relative to shock length and total compression needed. I'm the case of evos, 6" is all you need for travel in the 12k+ range.

 

I'm running 7" springs at the moment and love them but you think 6" is better? I will consider that now. Is this an autocross/track day suggestion or will it suit the daily driver as well?

 

With 10k front that might be a bit short there but the 12k rear 6" is plenty long assuming the spring perch can thread up high enough. FWIW, I run 6" front specifically to get the perch above the tire and a 6 or 7" rear spring with a tender. Rear length just depends which spring I'm running at the time, I change them frequently.

Depending if you have dual height adjustment on the shock or single height would also play a role. With dual height I run the tender rear to get more droop travel only so I dont get the sharp hit of shock hitting droop stop. With the single height I run a helper since its longer travel and the rate doesnt really matter.