I have no experience with dedicated summer tires on our X3 but our OE Scorpions were replaced at approximately 75,000 km and they had enough tread left to go to 95,000+ and still be safe, according to tread depth measurements on each tire. I wasn't planning to keep the car beyond 150,000 km so I replaced them early to get the benefit of driving on new tires during the time we will have it, primarily for reasons related to heavy rain and puncture resistance. I was very happy with the first set so I hunted out a supplier for an identical set. The tire store halfheartedly tried to talk me into something cheaper, a second tier brand that they had in stock. I told them to save their breath and order the Scorpion STRs in. Sales rep agreed he would do the same thing himself. The new tires are a little quieter than the old ones, but same performance, all as expected. I would do the same again.

I haven't used the Touring but had great service out of the MXV4 Michelin Energy tires on my X5. For dedicated winter tires, I put the X-Ice 2 on one vehicle, Blizzak WS-60s on another, and Dunlop Wintersport M3s on another, all in the last few years. All were great, especially the Wintersport M3s on the 535.

I would stay with the recommended size on the X3, 235/55-17

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue