Do Interrupter Brakes reduce braking performance from the main levers?
In an attempt to improve the braking performance of my wife's Ridgeback Voyage Tourer, which has always had very poor braking performance and a spongy braking feel, I'm planning to replace the current Tektro Oryx canti brakes with Shimano CX50 cantis running Swissstop Flash Pro BXP pads (in Shimano Ultegra brake shoes), which I've been very impressed with the performance of on my Hewitt Cheviot (compared to running Tektro TK720s), although part of that is due to the Shimano BL-R400 brake levers I'm now using.
My wife wants to continue using the Shimano Sora STI levers, even though I know they don't have the best mechanical advantage for cantis, but we're wondering whether to keep or ditch the interrupter brakes on the top of the drop bars. On the one hand, I know my wife does use these a lot (partly because they seem to brake better than the main STI levers...) and I think feels safer having them there, but could these be one of the reasons why the braking performance on the STI levers is so poor? Would leaving them out of the system give better braking from the main STI levers? If so, I think she'd rather have better braking on the STIs, rather than having both sets of levers.
NB: will also be replacing both the inner brake cables and outer brake housings with new genuine Shimano parts, as think they're pretty worn too, at least the outers are.