I have Q45 front/J30 rear calipers w/ 1 1/16 Z32 BMC and SS brake lines and the setup has solid pedal feel with little pedal travel. The only time it was "mushy" was when air was in the brake lines. With the larger BMC, you don't have to apply as much pedal pressure due to the larger cylinder bore. But your OEM BMC should work fine as long as you bleed your brakes properly, you may have to push a little further cause the Q45 caliper bore is larger than the S13's.
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