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I have asked this a few times over the years, and D1 inlets seem like they come in two major flavors for the F-body: stock procharger hat or the truck inlet hat. The truck hat flows better but both seem to start really holding up power around the 700 to 800 whp mark - or at least they did for me.
I just put on a D1x and none of my piping fits anymore (it shifts everything out about 3/4"), so I decided to do something about the inlet. It turned out to be a sizable project to get something bigger to fit.
Radiator had to be moved up about 1.5" and lower end out about 2". Lower radiator support bar was cut out and a customer 'claw' was made to create clearance. Radiator shroud had to be cleared in this area as well. End result, I'm pretty happy with. Turned out a bit like the truck inlet hat, just a lot bigger.
Taking the old inlet hat off on the dyno pushed up my dyno number by ~70+, so I'm hoping I can get most of that back. D1sc to D1x, I should see ~20 HP increase if I'm doing everything right. I am a little over 700hp to the tire right now, so these changes should park me right around 800hp - we'll see.
Finished up the under-plating this morning. Everything tucks up pretty well - a lot better than I thought it would with a 4" pipe around the corner of the radiator.
It looks like you have dealt with your bottleneck very well. 800rwhp is a great goal. I think you will reach it. I’m buying the fmic tomorrow. My twin 4.5” intercoolers hit a wall just under 700rwhp. Your post reminded me to include the truck inlet! I, too, am looking for an 8.
Since I am going from a 3.9 to a 3.7 at the same time will be interesting to see if there will be any improvement with the new hat.
Current specs: - D1 with 8.65 crank pulley and 3.9 pulley with an Aster bracket. Noticed a little belt dust when I removed the head unit to install the smaller 3.7 pulley.
On dyno session below was still using the Green Belt. Haven't switched to the RPM belt to test yet. Plan to test the RPM belt with the 3.7 pulley.
New engine is a TSP 370 with their 225 CC PRC heads with 68 CC chambers and 227/234 600-600 114 +4 cam. Idles almost as mild as my 2016 Z06.
Pics of the new designed hat assembly for the D1 on a 1998 -2002. IMO - not scientific - appears to be much more CFM capacity than the original hat and hose included with the D1 kit.
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Built a Better D1 Inlet Hat 
