Okay, I'll respond in order.
Tube: I bought it off eBay. It's a 3.5" diameter aluminum 16-gauge tube bent at a 60 degree angle. I just did a search for aluminum tube and it came up.
Heat tape: Yeah, I plan to wrap it in gold tape. Haven't done it yet. And yeah, Motons rule. This car has 700F/850R springs and with the Motons it rides nicer than my ZHP on stock springs and FSDs.
Water ingestion: If this car were street driven I'd worry about it, but it isn't, so I don't. Valid point, though.
Rotated MAF: I have heard that's the case, and until you pointed it out I didn't even notice that it wasn't oriented correctly. That's sad, because I have a mostly-stock '03 ZHP street car and the MAF is oriented differently but I never noticed it. It's just how it was when I bought the car. I'll fix it when I put it on the trailer this week to go get the aero stuff built.
'03 330i ZHP - Stock-ish
'02 330Ci - Megasquirted, E85'd and tuned by PFTuning.com