Got an extra set of nylon air filter for my 2013 Cbr400r (Sprint knockoffs from aliexpress), and did a ghetto version of the MWR mod to that filter and an extra airbox cover I bought. (Please brace your eyes for the unsightly result - I will be contacting a local cnc service to properly cut my used filter to size.)
Speeds up faster and the engine revving under load and with bigger, open throttle. It sounds weird opening the throttle in neutral (pinging sounds - stock exhaust) I am pretty sure you can do this with the paper filters and get similar gains, too. Do note that, similar to removing the snorkel only, this reduces engine braking (but obs more than just removing the snorkel).
Note that the modded air filter was not made properly and had gaps at one side, between the filter media and the frame. You can see white light peeking out on the bottom side of the filter. These were filled up with hot glue on the filtered side. I also made the mistake of trying to melt the glue even more by using a hot air gun... Filter media melted slightly, and I had to fill those up. Don't worry about the heat tolerance of filter material as I inspected the other one I'd installed and it did not melt due to heat from the engine (over 7000km since installation, including a one-shot 300km journey at night with short breaks and a 230km journey in the day with a lunch break).
Tried to install kspp velocity stacks, but am having trouble removing some stuff from the airbox.
Tl;Dr: CBR400R 2013; increased the open area of the air intake from 50mm diameter to a roughly 67x67mm square. Having trouble installing kspp velocity stacks.
P.S. Hot melt glue should be okay in this area. Some people use ABS for their velocity stacks (the KSPP stacks are ABS), and a local tuner said it'd be fine - won't get hot enough to melt.