You are going to get a ton of different answers. Each responder has found the setup that works for them.
Here's my setup after tinkering with it over the last few years

• Bento box: Specialized Remora, I have played around with Torhans, and Dark Speed Works. I have found that the Specialized Remora has the most volume and is the easiest to access.
• Behind the seat: During IMC last year, I opted to go with a single bottle behind the seat. However the TacX Saddle Clamp that I have been using supports dual cages as well. Super cheap and rock solid
• For a repair kit: i rolled my own with the Torhans VR Series. Took a soldering iron and a hacksaw, then used Sugru and neoprene from an old wetsuit to make a no rattle downtube flat kit to mount in that trapezoid.

• BTA: I have tried the Torhans BTA (both of them) was using the Aero 30 and its a really great product. However with the Alpha X, the Xlab Torpedo with the include computer mount is hands down amazing.
• Tires -- its a no brainer - to switch to the GP4000S. However, be careful with the 25mm tires. Check the clearances.
• PowerMeter : If you have a default P5 - it probably came with a Rotor crank - Rotor and Power2Max are made for each other. Unless you want to switch out the crank to something else.
• Computer: Whatever DC Rainmaker is recommending these days. Plenty of options out there.

Hope that helps