Just out of curiousity, what wheelset do you have? Keep in mind that upgrading the derailleurs and shifters won't really give you much of a noticable increase in performance. It will drop a very small amount of weight but nto enough that you will notice it in the real world.
What crankset do you have? If it is a heavy one you can lose a good bit of weight just by swapping it out for a lighter one.
Do you have plenty of riding clothes? Tools? Quality shoes and helmet? Those are places I would start if you don't already. Nice tools, a workstand, and a bike mechanics book are great investments.
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I may be fat but I'm slow enough to make up for it.