I recently installed my 42 dd downpipe for my 180q and wanted to share my opinion. I bought the race series catless version. Install was pretty strait forward, I decided to keep the stock dp in one piece so the hardest part was taking the old one out. for the 180q you do not need to completely remove the subframe, the stock dp will come out if you just unbolt the 4 main bolts and slightly lower the sub frame. I also unbolted the dogbone mount. Once the old one is out, the install is pretty easy from here on. Quality on the 42dd dp was amazing and the fitment was perfect.

After going for a drive the gains are instantly felt. it was like i had just chipped the car again. Power under 3000 is much better now where i found before lack a bit, and it pulls hard to redline and spools up much faster. One thing that i didn't expect was the change in exhaust tone which is much throatier and louder under WOT. I do however have a stock 225 exhaust on the car. Sounds close to my brothers mk4 with a Magnaflow catback lol.
So far my mods are just Unitronic stage 1+ and I'm going this week to have it flashed to stage 2+.
I have been driving it for about 300 miles now and the cell has yet to come on. I am very happy with the fitment as there is no rubbing, no vibrations or any weird noises. I did install Neuspeed dogbone bushings a while back.

 

I did the 42DD install on my 225 about 2 weeks ago. I did the race series too. It has an exhaust not silmilar to a G35(I love the sound). I didn't notice a HUGE power gain, but it is noticeable. I have the angle block spacer, and haven't had a CEL yet. You don't need to reflash to stageII unless you have a CEL from your o2 sensors. At least that's what APR told me. There aren't any gains from stageII. I kept my stock exhaust, and I think I might just leave it at that. I would like to have a SICK 3" cat back built, but I don't want it to be too loud. My next step is an MBC(should be here on Tuesday). I think the MBC install will bring more noticeable results than the DP.

 

I guess I'm not the only one that loves the new sound of the stock 225 exhaust and catless dp.
Unitronic recommends stage 2+ reflash as it would not only take away the cell but take advantage of the increased flow.
I guess the 180q dp was more restricted that the 225, because i notice a huge difference, seriously it was like i just chipped that car again.