The WonkyDonky kits are excellent so it'll all be down to that stumpy underlever.
So, as you say, you could just put up with that higher cocking effort or be happy to accept a lower power output by shortening the spring slightly. Don't think there'll be a massive scope for shortening, as I seem to remember that Dave's kits don't run lots of preload. Or, maybe, as you say, a softer spring.......