For my GXP, I did the K&N for a couple of reasons:
1. I work 10-12 hour a day 5 days a week, have 2 kids and just don't have time to design my own intake.
2. I make enough money that while I don't like spending $300, I can afford it if I save money for a couple of paychecks.
3. If you make it yourself, you do not get the help from K&N's lawyers if your dealer states that your intake destroyed the MAF, Cat, cam, piston's, etc. K&N's intake even has a handy 1-800 number to call if the stealership gives you hassle. I have actually used this service when a VW tech said that my K&N intake cause my cam to fail in my turbo VW, the lawyers talked to the tech and voila...warranty work with no more hassle.
4. The shield, while not blocking all heat, appears to do a fairly good job as after an hour of driving I can touch the clamp on the filter and it doesn't burn me, where as the clamp on the MAF housing does burn the hand...not one of my smarter moments.
5. Since there is very little support for the GXP, I have to believe that the R&D that K&N does is a little better than Joe fixit who uses PVC tubing and roof flashing to make an intake. Not banging on those that do it, more power to ya, but I am not an engineer, so I will leave the engineering to the people who know what they are doing. Just a change in materials, I think, can change the flow characteristics of the intake...but once again I am not an engineer, so that is just a guess.