if you go to 42 on the rear sprocket you can use the stock chain length. I think it increases your engine rpm about 400 more than the stock gearing.
I have stock gearing on my busa and I'm sticking with it when I change sprockets and chain soon. I want to be able to top the bike out and I don't want to be limited by shorter gearing. IMO the busa has good torque down low.
You'd have to search the info here or ask the experts but 'what chain length accommodates the widest range of sprocket gearing?' You can go 1 down on the front to achieve the same result as +3 on the rear (that is a guess). You can go several teeth down on the back to achieve the same result as +1 or 2 on the front. ...or you may want some combination between the two. Of course, the rear is a lot easier to switch out than the engine sprocket but it also effects the wheelbase much more and may be limited by whatever chain length you have to work with.
I went +4 on the rear with my ZX-14 and I love the way the larger sprocket looks. The increased mechanical advantage is fun at low speed. It is like running about one gear lower (~+500 rpm) so to achieve the same effect at higher speeds, I could just downshift. The bike feels more aggressive. It does change the character of the bike a bit. A few extra teeth on the back will not kill you. Taking off from low rpm, it is not a huge difference from stock gearing. I can wheelie better and I never flipped the bike. I can still do at least 180 and it appears likely I might hit the 11,000 rpm rev limiter at 190 with my current gearing. This is the 14. The busa has different stock sprocket gearing and probably the gearbox gearing is different too. Based on my knowledge of both bikes and my experience with +4 rear on the 14, I would say +4 on the busa would be a good choice for you. It will be somewhat snappier than a Gen1 14, it always was. If you don't go up enough teeth on the rear, you won't be able to tell much difference. I am sure +4 will throw your speedo off considerably. I use a SpeedoHealer on the 14 and it is dead on 0-90 mph according to GPS. It was too difficult to test above 90 on the street but I presume it's accurate all the way if it made it to 90 with no error.
Find out how far your wheel will be pulled forward and if that will effect tire size choices. I do not think +4 on the rear would. Look into chain length requirements. I have one extra link on my 14 with a +4 rear sprocket and that puts me at stock WB and allows me to use a 200/55 tire.
As far as switching both sprockets and chain or one sprocket only, it probably does matter over the long haul as far as longevity of those parts. Racers are swapping around gearing all the time though. Find what you like and then you'll probably stick to it. I would never change a chain and front sprocket just because the rear is worn out. I use two aluminum rears for one steel front and one chain. I have changed the chain and both sprockets after the front is worn out even though a good quality chain probably would go a while longer. How ever you do it, the parts will probably wear at a faster rate for a short time and then they are worn in together. You just adjust your chain tension as needed.
Finally, if the wheel is already off, it will be easier if you put it back on to break those sprocket nuts loose!
