Would there be any benefits from switching from a 2" piping setup to a 2 1/2" or 3" setup? I'm running a non-intercooled setup so I run straight from the turbo to the TB, not very much piping.

why don't you intercool it?

ever look in an e28 engine bay? :)
everything is packed on the passanger side -- The TB points towards the passanger side/exhaust manifold. All my intake stuff is crowded behind the headlight, and there's no room to manuever piping to the other side. The front spoiler can't be used for a FMIC anyway because of the mounting bracket -- even if I got around the mounting bracket, my oil cooler is completely in the way. Water/Air intercoolers seem to be some of the only choices us e28 turbo-ers have to do because of space.

I don't think you need a 3.0" unless you think that you will be running more boost in the future.

If you can and there is space I would upgrade the pipes to 2.5" all the way through to the throttle body.

If the throttle body is larger than 2.5" then maube go as large as 3.0" where you can fit it.

The TB is 3" - thats why I was asking about 3" piping. Since I have very little piping I figured I wouldn't see much of an increase in lag. But the question still remains -- What benefits would I see from increasing pipe size? Would there even be benefits?

Actually its no big deal to intercool it...Move the battery to the trunk and put the stock 745 intercooler where the battery was, duct it out th rear of the fender aka drill some holes...pipes can be easily made...no big deal...Makes a big difference...I have mine done that way but with custom ducting from the is spoiler fog light opensing and a special inner fender duct.

With the larger pipe it might be more efficient, cooler air temp, and less pressure drop.

By more efficient you mean -- more power?

Yes anything that makes the motor more efficient should produce more power. :)

Go with some W/A injection and 3" pipe and turn up the boost, but not too much.

I don't need W/A injection, with enough fuel the M30 engines can take some serious boost without intercooling. But the 745i wastegate limits me to only 10psi anyway.

I don't need W/A injection, with enough fuel the M30 engines can take some serious boost without intercooling. But the 745i wastegate limits me to only 10psi anyway.

Get a boost controller.....

*sigh*

I've explained this so many times. :)

The 745i wastegate opens at 6psi normally when it sees full boost from a boost line.
If I take a boost controller I can control the boost from 6psi to 10psi only. At 10psi the wastegate is forced open. The manifold pressure inside the exhaust manifold is enough to overcome the spring pressure of the wastegate and force it open. It was a semi-safety feature that BMW used to prevent overboosting. Even if there is absolutely no boost line attached to the wastegate, it will still open at 10psi. Scottie Sharp can work on these wastegates and increase the boost limit, but costs about $300.