So more surface area is always better, generally speaking double your cone area should give close to 5db given dame power as for seal vs ported efficiency is one thing but what makes ported louder is the tuning normally where its tuned gets a bump in output +-3 hz from tune however it then falls off pretty abruptly. That being said you will have similar output across all hz with sealed so you have a wider band of sound so to speak even if its not as loud. I have also heard that the bass is also tighter and more crisp going sealed over ported. How much more power is needed depends on the sub for a 500rms id say sealed could take 600 maybe 650 . The important thing here is clean power if you send clipped signal even rms can blow it.