I think a good fit is the best investment. However the picture linked says to me that the fit is decent so not totally wrong/whatever.
The next thing would be wheels, but you have them.
I went to a custom frame for massive fit changes (+1-2 lbs total added weight to prior bike, moved components over, got lighter/faster wheels, but frame/fork were heavier) and lost, compared to the same time the year before, over 30 lbs, closer to 35-38 lbs (190+ to 155). I upgraded to Cat 2 the year I lost all the weight, with most of my places save one on uphill sprint finishes in crits, racing in the same races I've been doing for 25+ years and unable to earn points to upgrade. Your power won't change much, meaning you're not going to double your threshold power etc. The only way to get your power to weight ratio down significantly (meaning by more than 1-2 w/kg) is to reduce your body weight.
So for me I'd look at fit, wheels (immediately possible), and weight loss (long term).
On the other hand if you want to spend money then spend money. I spend money on my car but I don't even street race it. It's just fun to fiddle with car stuff. Likewise I enjoy the equipment aspect of cycling but I don't have tons of money so I spend my money where I really, really think it counts. Custom frame. Custom stem (geometry wise). Light AND aero wheels. I got a power meter but use it more for data logging than for training. Saddle that works for me, ditto bars. Maintenance items. Component group etc who cares, if it doesn't break it's good.
Finally I'd get some long finger gloves, summer weight. The first time you skin 8 finger tips on the pavement you'll regret not racing in long finger gloves, at least if you type regularly at all.
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"...during the Lance years, being fit became the No. 1 thing. Totally the only thing. It’s a big part of what we do, but fitness is not the only thing. There’s skills, there’s tactics … there’s all kinds of stuff..." Tim Johnson