The only advantage to replaceing the stock tapered pipe with a pvc one would be if it was a larger diameter, in which case you'd have to cut a bigger hole in the airbox for it to fit.

The stock tube already pulls in fresh/cold air...

The K&N drop-in does give some small gains. Comapred to the sri not sure what would be better, as the sri is pulling in hot air from the engine bay. For the xr, doing a cone filter actually hurts the performance vs. the stock airbox(with paper filter) because of the hot air, but that's a turbo engine...)
The only real way to tell would be to take a stock car, and do a dyno with it stock, a dyno of the drop in k&n, and one with the sri. Which may have already been done...

As for the heat shield idea, you could wrap your stock tube with one.

The thing I find odd with the stock tube is the useless peice of foam they have blocking the intake of the tube. I prompty removed from my tube, the same day as I put in the drop-in k&n. Now I won't claim that it made my car faster, but it did seem to feel like there was a gain of some sort.

 

my injen sri made it faster. in another thread it was being discussed how hot the air the short ram is taking since you are moving and the air comes right in the front grills.....im not taking sides on that debate though.

 

ok, ive had the diy sri, and trust me its a good thing. just cut off the censor from the stock airbox and then attach the rubber throttle inlet tubing to one end and a k&n filter to the other side and expect mad results. but in the opinion of someone who went from ram air to cai i would rather try adn get a great deal on an aem cai. i got mine for pretty cheap and it was worth every penny. truse me. :-)

 

here is a pic of my cheap as sri and it made a huge difference - sure I have the RPW headers but when we did not have an intake the car felt sluggish in the lower RPMs when we hacked off the stock box and secured a K&N pod the difference was incrediable

 

pretty much the only diff. between that one and mine is they replaced that tube with metal pipe which i don't think makes a huge difference. 1-2hp at the most. maybe just alot easier to save the money and do it youre way.

 

I think it all depends on which filter you use... I have always been a firm believer in the K&N pods they breath that much better than most the sponge type if they get a bit of dirt in them when you clean them you will get most small bits of dirt out but not all of it so after time it starts to clog up and air can not move thru it ... at a recent car show we put my car on the dyno with stock paper filter and got 123hp atw we did a second run without a filter at all (safe enviroment so no chance of anything bad going into intake) and the dyno came back with 141hp

we also removed all of the plastic water protection stuff under the front bar to allow more air to enter the engine bay - the result is that were the alternator is and even the exhaust temps are now so much cooler it is incrediable, so this will mean that the overal heat soaked pod is now getting much more cooler and hopefully denser air. To improve on this we will at a later stage swap over the battery to a much smaller type (motorbike size) this will also allow more direct air to be pumped thru to the pod.

water damage it is winter here and my engine bay is nice clean and dry