They will last a long time if maintained properly and driven safely
What scoop is on it now? Stock WRX scoop? Stock STI scoop? Aftermarket scoop? You can't really make it smaller than the WRX scoop without some custom work but my guess is its the STI one on there. If thats the case, trade it for a stock WRX scoop with the proper splitter.
Hey I live in Houston as well. I have a WRX and I've been thinking of getting the larger STI scoop for looks and if any extra air for performance I doubt it would make a difference here in the summer but in the cooler winter the extra air will help. You'll probably notice a difference when it gets crazy hot again next year. If you have the 04 STI scoop and want to sell it I'd be interested.