From a performance stand point its common sense more air means u can use more fuel witch makes more power
ONLY IF the hole is too small to begin with. The engine can only suck in so much air. When you make the intake as big as it needs to be at WOT, there is no benefit to making it any bigger. You can't possibly know how big that hole needs to be unless you do a little 'rithmetic.
Also keep in mind that designs by manufactures are done so for many reasons such as market placement in categories such as insurance, wide variety of customers, etc. if they came out with the most powerful bike possible it would eliminate potential customers.
Insurance rates, to my knowledge, is based on the type of bike (cruiser vs sport bike, for example) and engine displacement, not horsepower. As far as powerful bikes eliminating customers, that's why Yamaha makes an FZ6, FZ8, and FZ1. The customer can choose how much power and how much they want to spend. So why not make each bike as powerful as possible (within cost constraints of course), especially if a larger intake inlet (which has no added cost) results in more HP? It's definitely a selling point when comparing similar bikes from other manufacturers especially in performance oriented bikes.
Everything is built to a budget and as we have a low cost bike, I highly doubt Yamaha used their best engineers or spent alot of time on this task.
You don't need your best engineer to figure out the optimal size of the intake. It's not rocket science. All I am saying is that if a larger hole truly has performance benefits and no downsides, and there is no additional cost involved, don't you think the engineers at Yamaha would have figured this out? I think we need to give them a little credit.
I want to share a relevant story so you all know where I am coming from. Years ago, I owned a Porsche 911 which I tracked and modded. I also belonged to a very active Porsche forum. We all thought the stock airbox was restrictive too and a lot of us tore it out and installed a K&N cone filter intake in it's place. More flow=more HP, right? Well, one guy on the forum, who tracked his car a lot, decided to get to the bottom of it. He took back to back dyno runs with only the following changes: stock airbox with K&N drop in filter, K&N high flow intake with stock airbox removed, and wide open intake (stock airbox completely removed, maximum flow possible). The results were surprising. The K&N conical filter high flow intake actually produced a few less peak HP than the stock airbox with K&N filter (not 10-15 more HP as claimed by K&N), and with no intake at all, it made less power than with either the K&N conical set up or with the stock airbox. So he sold his K&N conical intake and went back to the stock airbox with a K&N drop in.
So lesson learned for me is that there is more to it than just fitting the least restrictive intake that you possibly can and that the people designing the vehicle maybe do know what they are doing.
Maybe the FZ8 does benefit from opening up the intake, I don't know. Maybe the smaller opening delivers a flatter torque curve, or more torque at lower rpms. But I think it's a mistake to automatically assume that a bigger intake means more performance.
Sorry for the long post.