^^ guages are calibrated for the overall diameter measurement of your drive wheels. a 205/45 tire on a 17" wheel will give you the same overall diameter as your 15's. so that won't be thrown off.
as for how much it'll "slow you down", all depends on weight. naturally rotational inertia will change with a larger wheel, but as long as you keep the weight down, you should actually pick up some time with a lighter wheel, regardless of size.
As far as off the line traction is concerned 17's will be a little harder to keep from spinning due to the short sidewall, but that's only if you are at a dragstrip trying to get every 10th of a second. Normal everyday driving and even hard driving on the street you probably will see less wheelspin. You will definately see an improvement in handling. 16 pounds is excelent, anything with a weight in the teen's is good. 17's aren't going to slow you down at all.
As long as you keep the stock tire HEIGHT, you will be fine. Just because they're 17 vs 15 doesn't mean they will slow you down. The heavier wheel causes a more dramatic parastic loss, more turning mass, which slows down accelleration, you probably won't notice it. Infact some of the higher end forged OZ wheels are lighter than wheels 2 sizes smaller. If you get 17's, get 205/45 for the very best fit, the 2nd best is 215/40 (i run this fine). The extra 1cm makes a slight difference in traction and appearance.
you can get your speedo readjusted, yes, but there is no reason to you when you can run tires/rims with the correct width/height tires... Just get 215/40 or 205/45 and you won't have to worry about recalibratting it.
yah on my 17's I have 215/45 yokohama es100's....love em...I'm actually friends with a cop in my neighborhood so I had him follow me with the radar gun up to 90 mph and spedo was fine the whole way....isn't it fun to be friends with cops??? I think my rims are about 18 lbs and are just fine, but not too sure about the weight..hell check out the velox website, it might be on their...I got the vx-6rs...good luck and for the love of god keep u'r stocks cause u can't use 17s in the winter unless u wanna buy winter tires which is a waste of money, just keep the stocks...also the aftermarkets won't get all ****ed up from road salt etc.
PS: U might not have snow u live in cali...hmmm....yah u could get spedo recalibrated as derek said but it is a waste just get the right tires or just know u have to go 2 mph slower, oh no it'd save u God only knows how much just to remember u'r spedos 2 mph faster