I have a new top cap. Here it is on the the slightly dusty Scales Of Inaccuracy™, look:
The claimed weight, bolt included, is 2.9g - as you see, my kitchen scales aren't refined enough to prove or disprove this. Certainly doesn't seem to be overweight, at least.
This is an Extralite HyperCap - not cheap (no Extralite gear is, sadly), but I got this at a factory-special reduction because of "very small cosmetic imperfections". Works for me, because the one that was delivered (all the way from Italy) looks absolutely flawless.
So is it worth it? The cost and European postage? Well, it replaces a stem/bolt combo that weighed 17g, so I only strayed slightly the wrong side of the "£1 per 1g" cost/weight trade-off. Plus, I mean, come on, look at that hollow bolt... (don't look too closely at the design, I had no choice in that, it's the only one they do...)
So, swapping out a top cap to save 14g at a cost of €19... the transition is complete. I am officially a bike nerd.
P.S. The title of this post is a punning reference for readers of a certain age... because "top cap" sounds very much like this...