For a small amount, you can buy the IRC certificates of the good performers. With these certificates you can compare those performers to your own ship. A useful way to do this is to put all the information in a spreadsheet. The visible differences can shed a light on where their performance-gain could be found. You can use these insights as a starting point for your optimisation.
A single number-rating system like IRC means that your rating will represent an average performance of your boat in all circumstances. It is based on 50% upwind, 30% downwind and 20% reaching. How much this is true for you is depending on the courses that you sail.
Your boat can have a performance bias to its rating, for example: a Fast 40+ class or TP52 will probably be a lot faster then the rating predicts when going downwind in big breeze. So if you live in areas were there is generally a lot of breeze like San Francisco or Cape Town, a boat like that could be a good choice.
To make an older boat competitive, it often pays to use that bias. You will pay a penalty if the conditions are not favourable for you, but your chances of winning will increase when you do get the right conditions.
There are several general observations in IRC. Overlapping jibs get penalised quite heavy. Whereas draft is an easy way to gain a lot of performance.
The emphasis on rating or performance changes with the size of the boat. The break-even point for this lies somewhere around 45 ft. For boats that are longer the emphasis is more en more on performance. The smaller the boats get, the more important the rating is.
IRC vs ORC
The number of boats sailing under IRC and/or ORC are difficult to compare. ORC counts all certificates that they issued, including the trial ones that boats use for their optimisation, which IRC does not do.
IRC managed to open up the sail areas more than ORC. The general trend is that IRC is more performance-oriented then ORC. Which in the future might develop as a problem for IRC, since there are more cruiser-racers then thoroughbred race-boats.