The "attenuation volume" issue is precisely why the SIR sits way out on the front of the airbox, where we have seen it in F3, FIA sportscars, etc. - or so I've read. Remember that air is a fluid - it flows and eddies and stuff. I can see how it might confuse things, putting the SIR close enough in front of the MAF that it in essence creates a blast of increasing-velocity air from that little hole.

Particularly if I didn't have a bottomless engineering budget, I'd be tempted to build the biggest airbox possible, and use the SIR as the inlet. Put the filter between the SIR and airflow sensor, to leave things as much like the OE MAF is used to seeing them, absent the restriction, and count on the flexibility built into the system to cope with some of the variables that remain, short of sonic velocities being reached at the SIR.
K [/b]
You can't put the air filter between the sir and maf, the rule states all air must enter through the sir. If you put the air filter between the sir and maf you would be drawing in air that would not be going through the sir, making it illegal.
[/QUOTIt's on page 40 of the GT section of the GCR, called the "GTCS"
it says:

QUOTE
GTCS 17.1.2.F.4.i.10. Single Inlet Restrictors
a. The intent of this rule is to have a sealed system from
the Restrictor to the Intake Ports of the Cylinder Head.
All of the air entering the Intake Ports shall pass through
the specified Restrictor. Modification or addition to any
part of the Intake System that allows the introduction of
air into the Intake Ports that has not passed through the
specified Restrictor is prohibited.
b. The Engine Air Intake System must be fitted with an
aluminum air restrictor. The Intake System is defined
as an assembly of parts, including but not restricted
to: the Restrictor, Restrictor Housing, Ducting, Filters,
Air Box, Velocity Stacks, Throttle Body, Carburetors,
Manifold and Manifold Gasket up to the Intake Ports on
the Cylinder Head.
c. The Restrictor must be round in shape. The maximum
ID of the Restrictor is listed on the vehicle’s spec line.
The Restrictor’s maximum ID must be maintained for a
minimum length of 3mm. Restrictor mounting/placement
within the intake system is free, but must allow
accessibility for measurement. It is acceptable to have
some minor disassembly of the intake system to provide
access to the Restrictor for measurement. Measurement
device and restrictor shall be similar temperatures when
used.
d. Sealing the Restrictor from its supply of air must cause
the engine to stop within 4 seconds. This check is to be
made at an engine speed of approximately 2500 rpm.
The sealed airbox must withstand this test. Pressure
sensors present inside the intake system must be
disconnected during this check.
e. All GTL cars that have either an IR or SIR size (restricted)
listed on their spec line shall utilize an SIR for National
competitions.

So, all air goes through it.