Hi LCP,
Yeah, finding the rite speaker that appeals to you is quite a journey - just ask the likes of Affan, M30Revival, WJ and the rest here. But when you do and it's set up the right way, you'll know that all that hassle was worth it.
Okay, to answer some of your questions to the best I can - I'm hardly a Guru really .... still learning as much as you are too! ** Bear with the long winded response ...

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- how much kick panel cavity volume do you reckon on your setup? From the look of it and what my installer says, it's approx. 25% more space than the original panel and cover at the front stock location. At this moment, I can pretty much swap the UR6 with any other midbass unit with a mounting depth of up to 3 inch. Sadly, my upcoming SEAS babies needs more than that!
- relocation was quite simple as it was just a relocation of the original lever position to further infront. IMO, the whole thing was probably moved by just an inch or so. I'll try to find out more from my installer if any cables or other hardware was involved - none from what I recall really.
- footwell clearance was hardly compromised. If you look at the original pics that I'd posted in one of your earlier threads, there's hardly any obstructions to the throttle pedal and if you're driving a stick shift, heel-toe moves can still be executed swiftly.
On the note of the speaker test that you mentioned below, I'm just curious as to their testing methods and drivers used to verify their findings. Usually, the smaller drivers like a 5.25 inch mid will play much higher than say, a 7 inch midbass unit. This allows the tweeters to stay at a higher crossover point when paired with 5.25 mids as opposed when they are paired with 7 inch mids. However, the trade off as always would be in terms of low end extension where the bigger driver will usually have the edge.
So, if the 5.25 inch mid that you're refering to rolls-off at 160Hz, my guess is that it could be a narrow band driver which is probably more suitable for dedicated light bass duties - meaning that it would still need a pair of mid range and tweeters to complete the front stage. I've even seen 6 inch drivers that have an effective frequency range of approx. 45-160Hz which kinda limits their application in a 2-way system but works great in a 3 way system. The cone construction too is equally as pivotal in determining the rolloff and cone break-up points as each material ( paper, poly, fibre, metal, kevlar ) have their own properties and acoustic characteristics.
Regardless of what speakers you go for, its important that you take other factors such as set-up type (active or passive), instalation method and crossover points. Do remember that our in car seating position and door speaker location typically yields a 60-80 degree off axis performance which means further compromises are required. So assuming that you do go for the active route, you'll need to start paying attention to the crossover points of both your mids and tweets for optimum results. The general idea is that you should keep your drivers within the frequency response that yields as much of it's off-axis ability as possible.
As for the door pods / kick panel cavity, what exactly are your plans ? Are you planning to install larger drivers ( than the usual 6+ inchers ) or concerned about the breathing space for the drivers in the said mounting location ? Do remember that unlike subs, midbass units are usually much more efficient and enclosure space concerns are secondary to mounting/ install angles & positioning. But all things said and done, I always believe that scientific data, graphs and findings are helpful and essentially provides us with a clear and objective analysis of the subject at hand.
As Grissom of CSI says,
"Scientific data doesn't lie, it doesn't take sides, or care about the outcome...it's just data!" Ultimately, all this is still SUBJECTIVE as analysis and graphs really don't tell you how something will perform once it's in your car, but it helps greatly because we are trying to achieve the flattest response possible from our listening position, which is the challenge of car audio.
Regards,
VaN.