What spring are you talking about? AFAIK there is only one 'shifter spring' and that's the one holding the ball against the metal plate. Shortening that shouldn't make it stiffer, just too short to hold the ball. There's the pusher springs for the left/right force, but adding a couple of rubber bands will tighten that right up.
If you want to add a little force to the ball spring, here's a simple but effective way to do it, with the added bonus of getting rid of the annoying 'click' when you shift:
Get a cheap or old network cable. Cut about 0.75cm (about 1/3") of the insulation. Get the ball and push it into the cable insulation. Don't push it all the way in, just enough to make it stick. Get the spring, and push it in the other side (should be a tight fit). Put that assembly back in the shifter and enjoy.
On the subject of sequential downshifts...
Check the pot meter to the right of the shifter. It should fight into its slot really tightly, but doesn't. Get a piece of paper about 1cm wide, and 4 cm long. Fold it over a couple of times so you get a 1x1cm folded piece, and stick that in the slot so the pot gets pushed towards the back of the shifter (up). Downshifts should now work just fine.