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Offset is going to matter. Stock offset for the 21 x 10 wheels is 50. If the wheels you're looking at also have a 50 offset, the 1" wider wheel will protrude in by 1/2" and also out by 1/2". You need to keep the inner lip where it is relative to the suspension strut to avoid a clearance issue, so the new wheel will need an offset number 1/2" lower so it's pushed out - that means an offset of about 37 (50-13). Alternatively, you could use a 13mm spacer with a 50 offset wheel to accomplish the same thing. Spacers must have the hubcentric wheel center bore flange and that flange has to have an OD of exactly 71.6mm or the wheels will rotate out of round at speed - the lug bolts won't hold them well enough for you to not have vibrations. For the Cayenne, bolt pattern needs to be 5x130. With spacers, you'll need longer lug bolts by the thickness of the spacer. I like World Motorsports Ti Lugs and I have Adaptec Speedware make all my custom wheel spacers.
On my widebody Cayenne, I'm running a 22x12 wheel with a 25mm spacer, so my wheels are 2" wider than stock 1" in and 1" out and then the 25mm spacer pushes the inside edge back to the normal 10" wheel position, so the outer edge is 2" further out than stock to fill out the fender flares. However, my original fenders are unchanged and underneath those flares so my tires actually stick out beyond the original fender lines and never rub even in sport setting and high speed turns or bumps, so the Cayenne wheels never protrude up into the fender wells. I mention this because you won't have to worry about damaging the fender with the wheel / tire outer lip at or beyond the fender - maybe in an off-road situation it could with lots of suspension travel, but for road use, doesn't happen.
You'll want to use a tire size that keeps the rolling circumference very close to the stock one. I'm using Atturo AZ850s 315/30-22 which is within 1% of the stock 295/35-21 tire size. On an 11" wide wheel, you're going to need a wider tire - maybe a 305 or 315 to avoid the stretched tire look where the trim sticks out past the tire, which I don't like the look of and also makes it much easier to curb the wheels.
Those are pretty. Are you sure you want a staggered setup on the Cayenne though? That means you can't rotate tires front to back. The bigger issue is I believe if you have a flat tire on the front of the vehicle that the collapsable spare wheel won't clear the larger front brakes so you have to move the rear wheel to the front and install the spare on the rear. I'm not 100% certain of this though and it might only apply to the Big Red brakes of the Turbo S model with the 380mm front rotors or PCCB brakes with the 410mm front rotors.
Looking at the rear, the stock 10" wide et 50 wheels compared to 11" wide et 58 wheels means with the same offset you'd have the inner edge of the wheel 13mm closer to the suspension strut, but because the offset if 58 and not 50, 8 of that 13 mm is accounted for so the inner wheel and tire lip will be 5mm closer to the suspension strut - that's a little less that 1/4" so you need to check to see if you have that internal clearance. If you wanted to relocate the inner edge of the wheel where the factory wheel is, you'd need a 5mm spacer. With no spacer, the outer lip of the wheel is going to be 25mm wider for the extra 1" of wheel, less the 5mm remaining from the inside lip calculation, which equals 20mm further out or about 3/4". With the 5mm spacer, the outer lip will be 1" further out than stock - the thickness difference of the 2 wheels.
Looking at the front, the stock 10" wide et 50 wheel compared to 9.5" et 57 wheel, the 1/2" narrower wheel will be 1/4" further from the strut and 1/4" further in from the edge, but the 7mm offset difference pushes the wheel out 7mm. So at the suspension strut you'll be about 13mm further away (6mm + 7mm) vs stock on the inside and at the outer edge, you'll be 1mm further in because the 6mm difference in wheel width is compensate for by the 7mm difference in offset plus 1mm further out, which is nothing.
If you want the outer edges of the wheels and tires for the front and back to be flush, you need a 5mm spacer for the rear and a 24mm spacer for the front. Now, if you have to move the wider back wheel to the front if you have a flat tire, you have a 24mm spacer there that would also need to come off and be replaced by the 5mm spacer or that front wheel won't fit in the wheelwell for turns since it's now sticking out 2" further than the OEM outer wheel edge. This is the reason I went with a square setup for the wheels and tires on my Cayenne.
When you get into spacers of that larger thickness (15mm or more), it's much better to use the type where the spacer bolts to the wheel hub and then the wheel bolts to the threaded spacer. This prevent having really long lug bolts holding large loads. This is the type I have on my CTTS.
