Originally, the Lisp_Object type was a struct containing bitfields. This 
was significantly slower than using integer shifting and masking on many 
platforms, so long ago the default Lisp_Object type was changed to be an 
integer. Nowadays Lisp_Object no longer contains bitfields even when 
--enable-check-lisp-object-type is used, so this old motivation is no 
longer relevant.

For x86-64 there should be little difference nowadays when 
--enable-check-lisp-object-type is used. For other platforms (e.g., 
x86), however, I suppose there can still be a significant (though small) 
difference. So when compiling emacs for production, it makes sense to 
omit --enable-check-lisp-object-type, for the benefit of x86 and similar 
platforms.

When developing, the advantages of --enable-check-lisp-object-type 
outweigh the small increase in runtime cost, particularly considering 
that x86-64 is the most common development platform nowadays and there 
the runtime cost is insignificant.  So it makes sense to default 
--enable-check-lisp-object-type to "yes" if --enable-gcc-warnings is 
also enabled (which it is by default, in developer builds). I did this 
by installing the attached patch; comments welcome.

As a result, if you build master from a developer (Git-based) directory, 
--enable-check-lisp-object-type should now be the default. If you don't 
build from Git, I suggest configuring with --enable-gcc-warnings.