The idea that the better you are the harder the reed is an absolute myth. It's simply not true. If it were true then there'd not be so many choices out there.
Here's the deal;
your reed is only one part of a combination that includes your embouchure, mouthpiece, ligature, and reed. The first thing to consider is the lay of your mouthpiece. With an open mouthpiece we use a soft reed, with a close mouthpiece we use a hard reed. With a medium mouthpiece (what most student instruments come with) we use a medium (2.5-3) reed. That's why your band director suggested a 2.5 when you started out, because your mouthpiece is a student mouthpiece; not because you're not a good player yet.
The next consideration is the ligature. With a loose ligature it's easy to get a vibration so we can use a harder reed. With a tight ligature there's not as much vibration so we need a soft reed. In the middle (what your clarinet probably came with) is the medium ligature and we use a medium (2.5-3) reed.
Chances are, your 2.5 reed "sounds great" because you're using a medium mouthpiece and ligature and that reed is a medium.
A close mouthpiece with a hard reed will have a darker, more "woody" tone quality. Much as we expect in classical music. An open mouthpiece with a soft reed will be loud and bright - much like we expect from jazz and dixieland music. A medium with a medium reed will give a nice "all-around" tone quality. There are also medium-close, medium-open, very close, very open, etc... combinations too. They all relate to the type of tone you want to coax from your instrument.
You should try many combinations until you find your personal preference. Vandoren makes some very good mouthpieces, Rovner makes some great ligatures, and Vandoren makes excellent reeds. Pomarico makes a fantastic crystal mouthpiece (especially good for dixieland with a soft reed) and Vandoren makes a new ligature called the Klassic that is like a string-wrap that is great for a very classical sound.
None of these choices have as much to do with "how long you've been playing" or "how good you are" as they do with your own preferences.
Want proof? Ask the best player you know to put a #5 or #6 reed on their stock medium mouthpiece and see if their head turns purple from the resistance. It's not about "harder is better" - it's about "what works with this combination" and your own face.
Take your clarinet to the local music store and try some mouthpieces - you'll be amazed at the difference just a mouthpiece can make in your playing. If you want to try something cool with a medium mouthpiece and a medium reed combination - put a Rovner Dark ligature on there and see how you like that big sound.