You can overdo anything, the question is what's your limit.
I've seen people with HT's that have ten 18" subs driven by 20,000 watts in a space much smaller than yours. While every system needs 'headroom' - surfeit capacity that allows for smooth handling of broad dynamic swings in the source material - there comes a point of diminishing returns. Think Bugatti Veyron; here in the US you can occasionally find a highway with a speed limit in excess of 80 MPH, so what good is a car that can do 250 MPH then? If you own a Veyron you're certainly going to exceed 80 MPH from time-to-time, but you'll likely never be able to go much past 120. That means 50% of the cars potential is wasted 100% of the time.
At 2700 ft^3 you probably have good response with a pair of 18's and a pair of 15's. IB's are not known for high-performance though - more quality than quantity - so you might be able to enhance your viewing experience further with another pair of 18's, but at this point the updates are mostly evolutionary as opposed to revolutionary.