I have had 1 extract batch that had the twang. Since then I have switched to all DME and have made beers I would stack up to any AG out there. I do plan to move to partial mash, but only because I want to increase my involvement with the process.
As far as the yeast is concerned, it should not have any impact on the twang, the twang is mostly due to older LME.
And as for the monetary investment you have made, everything you have bought for extract will be used when doing AG (you'll still need that stock pot and fermenter, hydrometer, siphon, ect...). AG involves buying more equipment to supplement what you already have, not to replace it.
And now we go AG!
On Tap: Nadda
Planning: Extra Special Bitter