Good to see you are enjoying your windsurfing. That is the most important thing. You will find that as you lear to better control your small sail, you will look for new challanges.
It is normal that you wouldn't notice the difference between the old and new sails in light winds. They show their quality and best characteristics when pushed to the limits.
I also love sailing in light winds on my Phantom 320. I don't mind the low speed. I do race usually with a 8.5 or 9.5 sail and I have a 10.5 as well. Larger equipment is more expensive, especially with carbon mast and boom. I do find that I take a little time to get used to a sail and prefer not to change too much.
I no longer use any of my small sails since I am able to handle the 8.5 in strong conditions. You will know when you are ready to progress to larger sails, and if you never want to then thats OK.