I would think that spark plug reach is a major concern. What if I had 05 s60 2.5t. and I had a champion double plat 7975 plug. When measured next to a ngk g power bkr6egp the champion is about 5/16 longer. Is this ok or is it terrible?? I cross ref this champion plug to a denso and utolite plug. It stated that those two plugs were not for my car. I'm wondering if the champion plug is not for my car... although champion says it is.

 

Yes, reach does matter, especially 5/16 of an inch. It could interfere with the piston or even the valves. Too short and it will not ignite the fuel mixture.
I would not recommend Champion plugs, they fail quite a lot. IMHO Denso is your best bet.

 

+1 on getting the exact correct reach. That is important. Not sure if I agree on Denso vs Champion. Both have many styles - best to stick close to what the OEM design is and more expensive is not necessarily better (in fact the older turbos like my 850T run best on the copper core plugs). Its easy enough to shop online for the correct part number with an OEM design and correct fit.