I don't have that type of kick panel, since I bought my car with them already cut into it, but I wouldn't be shy to use it. I installed custom audio systems for 9 years some time back and still think kick panels are the best way to go. Front-facing music is always the ticket even if it's from below.
I'd recommend getting high db efficiency speakers to make the most out of the sound possible in those 4X6's. I am running Infinity Reference series and think the car sounds great. I believe they're 92db, which is far more efficient than similar priced competition and it's good sound quality too. If you plan on putting an amp on the speakers then efficiency is less important and RMS wattage will be a greater concern.
