In the near future i will be purchasing a VT/VX commodore.

I was thinking would it be good for SQ if I mounted these subs on the parcel shelf (with some minor modifications of course )

That way the bass goes directly into the cabin which could only be a good thing

Not for me.
A "free air" mounting of these things (loading into the boot) is NOT what they are designed for.

Boston recommend that their subs go into sealed boxes. They have been made to perform well into ported boxes (they used to offer complete sub and box combo's in sealed, ported and band pass a little while back) as well but they claim to prefer sealed.

Kicker do a nice "free air" sub.

it would be sealed. Im saying make sealed boxes attatched to the parcelshelf. i definitely wouldnt run these free air

By having them in sealed boxes on the parcel shelf would that lower the soundstage somewhat in the rear????

Would i need ambience tweeters up there???

I have some splits going in the rear doors maybe if i placed the tweeter higher up would that be such a good idea or would separation issues come into it

run them behind the seats firing forward. Definitely the easiest and best way when sound quality is a concern.

i ran bostons in a commodore very nice sounding speaker
which ones were you thinking of??

my sig....

2 Boston Pro 10" (10.5Lf)

firing subs directly into the cabin does not guarantee the best SQ, as i found out. indeed, SQ is great, but i also found no loss in SQ but improved low-end response with rear loading (as you'd expect). or at least, any 'loss' in SQ with rear loading compared with direct in-cabin mounting was not audibly apparent, especially without quick and easy swap around.

subbass is non-directional, meaning your ears will not detect where it's originating from (unless there's rattle/vibrations or higher frequencies being produced that can steer your ears that way). so the subs won't affect the stageheight.

however, speakers in the rear doors down low may lower stageheight. i found this in my car, so the new rears are going in the parcel shelf area. alternatively, as you suggest, having the tweets up high may avoid the problems, but likely introduce a new problem with 'separation' issues: treble sounding nice and high, but vocals and midbass down low.

good luck