I'm possibly missing the point here, but the way I read it, helper springs don't soften the spring rate one iota. The info. on the web clearly states that they are designed to completely bind with the unladen vehicle at rest and are only there to stop shorter springs from unseating at full travel. As I've cut mine down to the point where they remain seated anyway at full travel (just!) adding helper springs would only serve to raise the suspension by the thickness of the fully compressed helper spring. I'd then have to cut down the main spring by the same amount to reinstate the desired ride height, stiffening it even further.
I'm with Dave C - I'll try it for a while with the standard springs cut down and experiment with softer springs if I'm not happy with the ride at a later date. I've been out as a passenger in Phil J's BRG Spyder - 2500 straight six, standard Vitesse springs cut down by 90mm and a thicker front anti roll bar - and I don't recall the ride being particularly harsh. But then I did have a huge silly grin on my face for at least an hour after he left.
