switching to a ported box

I currently have a 12 inch Solo Baric L5. I received it very cheap from a friend; it is attached to a Kenwood KAC-7402 I think. (I'm at work and cannot verify right now) It’s in a 1.5 ish sealed box I built myself getting 500 watts RMS from the amp. I think it sounds good. I built the sealed box because I was told that ported boxes always had distortion and poor response for songs with fast beats. After reading around I found out that wasn't necessarily true. Other quick background before I jump into my question, it’s going into a Honda Civic 2008, factory stereo still installed. Amp is getting its sound from speaker level inputs. Not the best setup but good for my limited bank account etc.

So now the questions I have:

1. Based on my readings it would be advantageous to move to a ported sub to get lower end frequencies and get the SPL up a bit. Is that true? Or should just stick with my Closed --- properly tuned ported box will always be louder than a sealed box.

2. My amp doesn't have a subsonic filter, where I am under powering the sub by 100 watts RMS will that be a big deal? --- yes, sub could potentially play lower than tune and damage the sub mechanically.

3. Could I build a passive filter and put it before the amp on the speaker line to filter the subsonic frequencies? (I’m no stranger to electronics, just car audio) --- if your head unit doesn't have it, your cheapest route would be an equalizer with it or an FMOD.

4. How important is it to get the physical dimensions into winISD when planning boxes? I have not been able to find the physical dimensions just the T/S. --- dimensions of the sub play a big factor into displacement

5. It might be that I’m still not figuring out winISD yet but when looking at the excursion it only shows like 1mm max. What am I missing to show correct excursion?  --- not sure

I think that’s all I have. I have read a lot on these forums and have really appreciated everything I’ve read and I hope these questions aren't easy things I just overlooked when reading through the forum. There just questions I have developed during my course of reading.