Arthur Hagen has offered us the following clue :
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There is a "genkernel" utility that analyses your system, disregards
that information and then enables everything anyhow, and you end up with
a HUGE kernel with tonnes of modules, just like SuSE and other distros.
So even though you compile the kernel, you don't have to answer lots of
questions, and can look at it like installing rather than compiling
(although it does compile it for you).

Using genkernel is, IMHO, to overlook the strengths of gentoo -- that
you can easily configure to fit your particular system. If you have a
single CPU without hyperthreading or multi-core, disabling
multi-threading will give you a noticable performance increase. If you
don't have SCSI or Bluetooth, you don't have to compile and poll for
those at every boot.
Instead of a 50 MB kernel with an equal amount of modules being pulled
in whether needed or not, you can get a 3 MB kernel that does it all,
quicker, and with the saved memory available for disk caching. This
does improve system speed.
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Mmmm, I use genkernel but I don't understand your point. Why should the
kernel be bigger ? Mine is 2,7MB. I select my desired options before
compiling, exactly like you say, modules, features etc. For me, genkernel
is just a facilitating tool for compilation with genkernel --menuconfig
--udev all