a few more ??? on my 99 yzf

My riding partner used to have that bike. Not sure about the jetting. If the seller lived in Phx. I would leave the 175 in unless you gets some pops and crackles on de-celeration or it coughs a bit on the extreme top end. No lid on the airbox.
But if you are going to put a spark - arrestor silencer on, you might have to adjust your jetting again, but probably not.

It's somewhat normal to require a slightly leaner main jet during the summer. Why? Because warm/hot air is less dense than cool/cold air. If youare using the #180 main during the winter and the engine runs fine...as the ambient temperature rises, it's normal that the engine becomes sluggish at sudden throttle changes and blubbers on the top RPM range because the engine is now running slightly rich due to less available air. To compensate, usually a slightly leaner main jet will cure the problem. Try a #178...and test for wide open throttle runs. If it still blubbers a bit...go to the #175. The goal is to get the engine to run the best with the richest possible main jet. What I have found is that once I get to the correct jet, the engine really comes alive. Also, I use VP C-12 fuel. This fuel typically runs on the rich side so compensation is in order. Oygenated fuels are the opposite. Since main jets are very easy to change, experimentation should not be a problem.

it never had an airbox lid.

the spark arrestor from the wr should be a bolt on, but unless you have access to a cheap used one, i would bet that the cost will be higher than aftermarket. i know the weight and the performance will be less satisfactory. on the other hand, it will be nice and quiet. if that's not an issue, you might want to consider one of the units like pro circuit or white brothers that has a removable screen section.