A 10" sub with 9.3mm xmax is probably not going to be any louder that that Arc 8
The Arc is most likely to have better SQ but I have to say most likely because I know exactly how the 10's sound and they are very good indeed. I have not heard the eD 10 but every other shallow mount 10 I have tested(many) just could not get loud enough or had good enough SQ for my tastes.
Arc subs and IDQ subs are the closest match, IDQ has a slight edge in SQ, Arc a slight edge in SPL.
The ID8(not the same as the IDQ8) is still a dang fine sub with really great SQ for its cost, I know since I have used them and have a couple new ones on the shelf.
I chose to use the Arc 10 in our underseat sub systems for their high SQL potential in a small enclosure but the IDQ would be a sweet second choice. I looked at many many subs before going with this one (we are a licensed OEM manf for using Arc and ID subs)
We do not build many underseat units now due to not having much time as our regular business is growing so rapidly but I am going to use one in our FXT very soon. BUT, if you have rear seat passengers on long trips it is a bit uncomfortable to not be able to put slide your feet under the front passenger seat. I am going to mod ours to deal with that and give up a bit of low end extention but after calculating the numbers I decided I could live with it more than dropping down to an 8" sub which would still give up extension and SPL over a 10 in this particular situation.
As for the comments on which size is best for what type of music, it is not so cut and dried, far to many variables to consider, small subs can go very low but generally have to have very long excursions and used in mulitples. Large subs can be very and punchy if utilized correctly. The real problem is very few shops and individuals have ever really heard a properly setup bass system and thus really do not have a proper understanding of what real bass can and should be, sad really
Rick