Diff leak and aftermarket (or not) exhaust tech question

[ultimatequinoxe]

So to my amazement my late 2000 (T3?) GS300 passed the MOT with its very aged and knackered exhaust. Two advisories, webbing of a seat belt and "oil leak pinion rear diff". So two questions if I may for your thoughts, experience and knowledge :) 

1. Theres no Diff whine, and though wet, I'm not getting any drips. And to be honest I've not had a leaky diff on any car before. Is this common? Tough to fix? Indicative of anything major (other than the diff running dry of course)? (cars just coming up to 150k miles)

2. Exhausts. How much difference to the sound do the "resonators" and maybe third cat make internally? I don't want to lose back pressure anyway, but I see/read that people who have fitted stainless steel systems without the "resonators" on the middle/Y-pipe are getting a lot of drone inside. What needs to stay in some fashion to keep the internal noise level as close to stock as possible?

I ask because I still want to replace the system from the front to the back (not the manifold and 2 front cats though, they're okay). Standard systems used are £300+, third party new totals around £400 or so. Stainless steeel fitted new, cheapest quote so far is £390 (I've had loads of quotes). However, thats almost certainly without the resonators/mufflers on the Y pipe.

Last option and most likely one is some shiney tail pipes from the USA (those plus shipping works out cheaper than UK fitted ones). All I want is a healthy system in the middle (third cat/front pipe and middle Y-piece) and shiney non-OTT tail pipes that just give a little more external tune for the lovely 2JZ. Seeing as standard rear boxes are pushing on £200 a piece new, I can get a pair of stainless steel ones for about £280, or for £340'ish I can get a Manzo cat back (Y-piece and rear boxes) however the Y-piece has no "resonators" and tempting as it looks, I fear a terrible noise inside and possibly a drop in back pressure I'd rather not have.

Sorry for the waffle, but the whole picture might save some guess work for you :)

(Images of a standard system and a Manzo cat back for reference about what I am waffling on about)