One effective method speed shops have found to let sled engines breathe easier is to install several of the adjustable TPI valves.  Available in round upright and flat versions, they provide a method of varying the amount of intake surface area.  This has proven to be an effective way to allow more unrestricted airflow into an airbox, but the key here is the adjustability.  When the valves are closed, your intake remains bone-stock.  As you open the valves, the airflow is increased, and continues to increase all the way to the point where the valves are wide open.  This is a very valuable tuning aid when calibrating a system, as adding more airflow requires more fuel to maintain safe fuel-air ratios. 
     Several speed shops have reported nice gains using TPI valves.  Our own testing here at SnowTech revealed a nice power gain of a couple horsepower on an otherwise stock SXViper.  Dakota Performance has seen gains of 3-5 HP when installed on Firecat systems.  Cudney Racing reported gains of up to 5 HP when TPI valves were installed into the airbox of an MX Z 800.  Wild Horse Performance in Preston, Idaho has seen some impressive gains when installing TPI valves into the airbox of RX-1 models.  Again, tuners can vary the amount of the opening to match the set-up to the conditions, or to allow more intake as the conditions change on a non-compensated system.  Contact Thunder Products at 320-597-2700 to order or to learn more about the adjustable TPI valves.  One valve per 200cc is the base recommendation, so four valves are a good starting point for an 800cc engine.  Start out with the valves closed (stock) and slowly open them uip and track you plugs, pistons and exhaust gas temperatures.  Make changes as needed, and the next thing you know you'll be needing to add some clutching to make use of the extra power!