The performance deficit was not too much of a problem, especially for what superiority later Spitfires enjoyed.
RAAF pilots were known for bolting the 30 gal tank on the plane during 1945, but then again, they were staying out of heavy combat zones.
In the game, every little thing hurts performance just like in real life. If you want the most out of your bird, drop that tank when empty. Taking the Spitfire VIII can usually be a healthy alternative due to its slightly longer range than the IX. So if you're going anywhere where the IX will need extra gas, try just taking an VIII.