If you significantly step up the front rate, you would need to step up the rear rate an equal percentage, assuming your car was balanced to begin with.

However, with tubular bars the rates are slightly different as it is contigent upon wall thickness. A 1.25 or 1.38 tube bar will have a similar rate to a 1.125 solid bar. The advantage in going with the tubular style is you can reduce weight considerably without giving up rates.