Anything can be done. No doubt about it. But a forum-friend of mine, on another generic car forum, went down this road with TT on an E60 M5. It ended with him building this in an E46 M3 instead, because of two main reasons: no space where the turbos should be positioned for optimum performance and a "mint condition" low mileage E60 M5 is too expensive to just chop into pieces, hoping to get it to perform well. You'll have problem getting it sold and getting it through the mandatory yearly inspections and that's a big and expensive risk to take on, for most DIY:ers. When you are talking about tuning companies, it's quite the opposite. The number of high income over-class E60 owners are getting fewer and the cars are not expensive enough, to invest another $20-30k in performance parts.
The build cost of his S85 TT + gearbox and rear axle was approx $25-30k in just parts and machining. All the manual work, besides the ECU tuning, he did himself. For this money, he got approx 1400 chp on ethanol and 1100 chp on pump gas, good driveability and presumably decent longevity, as no costs were spared on the parts. Only top notch. So, it surely responds good to turbo charging, but it's the market and the fitment problems, that's been keeping the tuners away from our S85's. Not that it would be a "weak" engine that don't respond to tuning.