Yes, the way I imagined it the Wye would be flat (of course relatively flat -- it still needs grade). I would think that would eliminate any noise also but then I haven't put in plumbing where I had such a long vertical drop. I usually can only go up one floor at a time then offsetting because of the house framing. If you can't make that arrangement, how about this -- you move your combination you have down the line as far as you are able, put a 45 up high, then another 45 turned down into your combination. The way things flow, water (and stuff *s*) tends run down the inside of the pipe and not directly in the center. I'm hoping the extra 45s can help slow one side of the flow or another. Is the noise just as evident when you just flush it and having not used it (ie, just water?)

I think if you take the time to analyze the facts your answer will be a no brainer. Fact- A kitchen makeover is without question one of the largest and most expensive projects you can undertake in your home. Fact-The sink is without question the singular focal point of the kitchen. Almost every time you go in the kitchen, regardless of what the reason, you will ultimately end up at the sink if only to grab a glass of water. Fact- If you have your heart set o a 10" sink and settle for an 8" sink it will forever be a source of irritation and contention, one which you will confront dozens of times a day. Fact-If you are changing all the cabinets you will already have easy access to the wall and changing the drain line is no big deal. Fact-If you install the 8" sink now, then later decide you really want the 10" sink it could prove nearly cost prohibitive to make the necessary changes to the drain lines. Now let us consider what would be required to lower the drain while you have access to the wall. In a best case scenario when the fixture arm enters the wall it is connected to a vertical line by means of a sanitary tee. In that case it would be very easy to cut the line, lower the tee to the desired level and reconnect the line. In a worst case scenario the waste arm enters the wall then turns horizontal and runs through the wall to a point were it connects to a vertical stack. The question then becomes how far does the horizontal line travel through the wall? Under the IRC a kitchen sink drain line is an 1-1/2" line and it is allowed to run a maximum of 6' from the trap weir to the vent opening. Allowing that there is generally about 12" from the wall to the trap that means that the line could run up to 5' horizontally in the wall. Under the UPC we are required to use a 2" diameter drain line but the total developed length from trap weir to vent opening of a 2" fixture arm is limited to only 5' so we could anticipate a maximum of 4" horizontally in the wall. In all probability the portion of the wall that you would need to open up will most likely be covered by the cabinets when all done, therefore when you open the sheet rock it would only require a simple patch with one coat of mud & tape to prevent bugs from entering. Not a big deal here. The question then becomes how much should you lower the line? From your preliminary calculations you determined that you need approximately 2" however, from my experience it is much easier if we allow ourselves plenty of room, therefore I would lower the arm at least 4 or perhaps 5". Keep in mind that code will allow up to 12" vertical drop from the finished sink opening to the water level in the trap.

Going into a drain line with a toilet above is not a god idea. The back of my mind says you cannot do it under the code.

Internal drainage doesn't stop the water from damaging your foundation and undermining your footings. Unless there's absolutely no alternative, eg., there's only a foot between your house and a neighbouring house, it's a poor choice. Absent the known water problems, this could be taken as mild settling. Indeed, I have friends whose home leaned over several degrees but was still deemed by a structural engineer to be stable. If you're concerned, the only way to be sure is to have a structural engineer check it out.