I ran an 18x9.5 +55 and the wheel was quite sunk into the fender, so I don't see how 15mm would stick it a 1/4 inch past the fender.
Regardless, it is in no way senseless to run a 235 on a 10". I can understand wanting a bigger tire if you're boosted, but for a stockish IS you really don't need anything bigger than a 235 out back. And if stretch is your concern then please do not start this argument. A friend of mine put 215's on 10.5" wheels with no problems... It may not be your style, but it will fit without a problem.
-Dude, take your honda game elsewhere man. If you're going to sit here and preach about it's okay that's fine but don't sit here and advise members that it is safe. I know it will fit but that is besides the fact. Anything will work and fit with alot of fagina grease. Why purchase wheels with the incorrect offset and width and have to do the whole nine yard to make it work (camber adj, fender pulling, fender rolling, severe tire stretch, etc.) . If you are going to spend the money on wheels spend it on the right wheels. Offset, backspacing and width were developed for a reason. And throwing a 215 on a 10.5" weren't one of them.
-I guess you're new to the IS world. The 9.5 +55 thing is old school and it was well established that it work, years ago.
-You're right Miss_IS300 ran 18x9 +25 up front with 225. Now ask her if she had any rubbing issues. The rim itself extruded pass the fender, even with a stretch tire it will still rub.
-As for your friend running 215 on 10.5, that's fine. You do what you want to do but don't advise others to do the same "thinking" that it's safe. My only suggestion is to contact your wheel manufacterer and tire manufacturer on the tire to rim width recommendation.
-You're sort of contradiction yourself if you ask me. What's the point of running anything above a 8.5" wide wheel if your motor setup isn't even warranting such width. Stick with a 8.5" or lower and run the 225-255 that you want to run without having to spend additional money to the "works"
-Just FYI a MM will make a huge difference.
-Experiment with tire sizing? What's the point of asking then. Just buy some 215, 225,235,245,255,265, etc. and see what works.
You're too quick to jump to conclusion. Many of us, older members, have experienced with many different width, tire size, wheels, etc. I have ran 5 sets of wheels on my IS in the past 3 years and all were nothing below 8.5". Phillzspeed currently own two IS with two sets of wide width. Hmm, I only wonder it's the older members who tend to preach against stretch tires. So Import-tooner, it's your choice on whose advise you should take.
Until you have experience personally with your own hands, please don't suggest that it will work. I am currently running 18x10 with a +25 out back with 275 tires. I am in the process of taking those 275 out and even running 285-295. My wheels extrudes 1/2, maybe more, pass the rear quarter. Not the lip, the quarter panel.
Also, until you have experienced with 275-315 tires don't consider yourself knowledgeable in the wheels for the IS.
