Then again my Model S has the P+ suspension, with slightly different geometry from other versions.
You’re in the X forum...the S came with a maximum rear width of 9.0 and used a stock offset of 40mm w/ 265 tires. The X came stock with a rear width of 10.0 and came with a stock offset of +35 with 285 tires, so not exactly apples to apples. With that said, I just realized the OP said he had a 9.5" stock rear wheel...
@HugoBoss the X's didn't come with a stock 20x9.5 wheel. They had a 19x8.5, but the 20's and 22's are 9.0 in the front and 10.0 in the rear. They should be the same as your tsportlines, but your Tsportlines will have a +40 rear offset vs a +35 stock. If you're getting a stretching look, it may just be a combination of how the tire sits along with the wheel design.
I would recommend you double check the specs of the wheels and tires. Another option is to get the same spec, but a blockier tire type or even one with larger tread widths. As an example, the stock contis were designed with a tread width of 8.9" where as the Pirelli Scorpion Zero All Season Plus has a 9.1" tread width despite both being 275/45-20 tires. That extra width would be equivelant to 5mm, or a 280 tire. The Hankook Ventus S1 EVO2 is 9.4" and equivelant to 12mm extra on tread width. *All specs are according to tire rack.
If you're still dead set on going to a 285, what I do is I use
Rim & Tire Size Calculator. Custom Offsets to get some visuals. After you input your data, click on comparison table on the bottom and it will give you a readout of the changes. For what you're propsing..
Comparing a 275/45/20x10, +35mm vs a 285/45/20x10, +40mm, direct from that site:
Suspension: Tire is 10 mm closer to suspension components. Acceptable for most cars"
Fenders: The same
Wheelwells: Will fill out the wheelwell by extra 5 mm. Acceptable for most cars
Comparing that same setup to OEM 285/35/22x10, +35mm:
Suspension: Tire is 5 mm closer to suspension components. Acceptable for most cars
Fenders: 100% clear (leaves even more room than OE)
Wheelwells: Will fill out the wheelwell by extra 3 mm. Almost no difference