You should always keep your cap the same rate it was originally. Otherwise you are just increasing the pressure on the rest of your cooling system which might or might not be capable of handling it.
New welded aluminum radiator should handle the extra pressure. I've bypassed the heater system so I'm not sure of any other parts that couldn't handle a little increase in pressure. Doesn't increasing the pressure also increase the boiling point?
Correct me if I'm wrong but I don't think your cap determines your boiling point. The antifreeze mixture controls that more than anything. Your cap only controls the point at which your system opens up to allow the coolant to boil over into your overflow bottle. If you've bypassed your heater, etc then go for it... however I have to ask myself why you would want a higher rated cap?