P0301 high RPM misfire

[ISpy]

2003 MT, 70k
New: plugs, coil over plugs, oxygen sensors, exhaust header-to-tail pipe, injectors and seals,.air filter, oil, oil filter.
Cleaned MAF, throttle body, air box, PCV.
No oil burn (1 cup per 1400 mi), no leaks, clean OEM motor
Vacuum steady at idle and steady when steady on throttle. Zero waver. 21.5 in-Hg at idle.

Okay given that, my issue is that only under wide open throttle (WOT), only accelerating, only high in the range (above 6000 RPM) I get a slight sound and feel of a misfire.  The CEL will flash and if I continue on through the next gear I get a solid P0301 (cylinder 1 misfire). I purchased the car 9 months ago and didn't discover the issue for a month and assumed it was plugs. Plugs were original and clean,  looked good. Swapped around, nothing changed. Swapped around coil packs. Nothing changed. Used lucas for 1000 plus miles with high octane high detergent, nothing changed. Swapped around injectors, nothing changed. OEM downpipe flex section broke, replaced with full PPE header, high flow cat and dual muffler. Discovered O2 sensors were Bosch. Replaced with Denso. Nothing changed with CEL except now I get P0420 intermittently (inefficient cat). Decided at 70k I would replace coil packs. No change, P0301 remainsonly under the humping it scenario. Swapped around coil packs and plugs, no change. Got new injectors. Interestingly enough the CEL flashing and slight hesitation moved from around 5800 to 6300 and it was barely perceptible. Replaced injector seals and swapped around the new injectors, still high RPM wide open throttle P0301.  But I rarely go there as this is my daily driver...  So I then figure it could be a pressure related vacuum leak,  intake manifold gasket, or fuel delivery issue.  Vacuum testing revealed an idle vacuum of 21.5 in-Hg. High side but solid. Vacuum responds just as it is supposed to. Under acceleration it drops to around zero but once I steady the throttle it holds lower vac but steady.  So to me that eliminated a vacuum leak.  Now I am back to fuel delivery and guessing.  

Does anyone have any suggestions?  If this did not require WOT under load above 6000 it would be easier to diagnose. Like many of you I am a shade tree mechanic and my knowledge of the intimate workings of the electronics is what I get from the Web when I know how to phrase it and then read and guesstimate. Thanks for your suggestions.  If nothing else this is a learning experience for us.  Thanks.