make_immutable should make accessors faster through 'inlining'.

Not as I understand it. I believe accessors are always inlined. Rather, make_immutable speeds up object creation. Using your test framework with the following tests

my %tests = (
    Moose     => sub { $x= MyClass    ->new },
    MooseImmu => sub { $x= MyImmuClass->new },
);
[download]

Yields:

              Rate     Moose MooseImmu
Moose      15360/s        --      -94%
MooseImmu 261482/s     1602%        --
[download]

Update:

I've been grepping pods...

Moose::Manual::BestPractices:

The "make_immutable" call allows Moose to speed up a lot of things, most notably object construction.

Moose::Meta::Class:

$metaclass->make_immutable(%options)
...
Also, since Moose always inlines attributes...