What is the problem is when changing a spring to change engagement, when everything else is correct, causes the whole shift to change. If you go up 10lbs on engagement pressure, you are also effectively decreasing the force of the weight all the way up through the shift curve, progressively lessening the effect as it gets closer to full shift out.
a 5lb jump, may not effect it much, but is still, 5lbs at shift out and 2.5 at half shift out. Be ready to adjust some other stuff if your on the money right now and change spring for engagement