it's hard to do good & cheap with ecus, nistune prob most eco, link g4 is good ,does boost control & doesn't need z32 afm so in the long run not too bad if you're going to go further, there's a few reputable tuners on here so see who's in your area and they'll be able to recommend for your goals
as above, weak coils prob fouled old plugs, just change them, haven't heard of those coils, maybe they're getting weak already, go for .8 plugs
(this isn't forced induction performance related haha)
This!, I recently sold a VE SV6 & i've many DE daily drivers, unless the sv6 has something big time wrong with it, it will dish you a beating, an exhaust that big on an NA will just be a megaphone
this, the shim will only tighten the lsd action in the cheapest way, apart from welding it haha, the metallic ting sort of noise when put in reverse will be backlash which is just from wear, I'd prob just put up with it until diff upgrade time