On my vehicle with a transverse mounted v6, I discovered that the bank closest to the firewall was most easily accessed from underneath the vehicle. I was surprised to discover more room back under the V of the engine than I had thought. Maybe the same is true on yours.
For vacuum leaks, I’ve seen a neat trick on YouTube: use a fog/smoke machine. On a cold engine that’s not running, feed the output of the machine into the brake booster line. After a few minutes, if there are any major leaks, the fog will start escaping into the engine bay. For minor leaks, if the fog/smoke doesn’t help, warm the engine up and spray carb cleaner at gaskets/mating surfaces and anyplace that a vacuum hose connects to the manifold: any leaks should suck the cleaner into the engine and change the idle.
Last, as far as your decrease in efficiency: if you’ve up-gapped your spark plugs, check the resistance of your spark plug leads: any resistance prevents a strong spark from forming. The lower the resistance in the wires/leads, the bigger the spark and the better the fuel in the cylinder burns. TracyG and I swear by our Ganatelli Mpg+ zero resistance wires. If they’re not easily acquired for you, look for the lowest you can find by taking a multimeter to your parts store and measure the longest wire from each kit/set they have for your engine. There is enough resistance in your spark plugs to keep your ignition coils happy, so get as much of the energy they make to the place it matters most (the spark gap) rather than wasted as heat in the wires.
Another note: you mentioned a high flow air filter. If that means you can clean/re-oil it, consider this: “More” Air makes the computer deliver more fuel. Further, the oil droplets can coat the wires of your MAF sensor, throwing air measurements for fuel delivery off.
Make sure you perform a full re-learn for the computer to figure out that you’ve made these changes.