I keep hearing the back pressure question and have to wonder why it is a question. The object of good headers and a cam with overlap on a racing or street performance engine is to clean the by-products of the last combustion cycle out of the engine cylinder. The header length is designed to create a negative pulse, or vacuum, at the exhaust valve while it is open. The cam overlap that allows both intake and exhaust valves to be open at the same time for a short time allows the header negative pulse to draw air through the cylinder briefly to clear out all of the waste gas possible. The rule on any engine is: EXHAUST BACK PRESSURE MEANS THERE ARE RESIDUAL COMBUSTION BY-PRODUCTS IN THE CYLINDER WHEN IT STARTS THE NEXT INTAKE CYCLE. Those residual by-products take up space that could be used for a new combustion charge, therefore reducing the available volume of new fuel/air mix. This means less power.
Some engines will normally have higher back pressure that is unavoidable, like ones with turbos on them. The back pressure is the result of the pressure needed to spin the turbo compressor wheel. This problem is somewhat offset because the turbo forces air into the cylinder on the intake stroke, helping reduce the affect of the remaining exhaust gases.
A normally aspirated engine with a long exhaust system and restrictive mufflers will have some back pressure. It is an unwanted result if you want performance. The less back pressure you have on a naturally aspirated engine, the more power you can get because the new incoming fuel/air charge is larger because it has a more empty cylinder to fill.
You can even run a diesel or other engine with no exhaust manifold, but the advice about the system being extremely short allowing cold air to the exhaust valves on shut-down is correct. Normally on a truck or car, one runs the system back past the driver resulting in a system long enough to protect the exhaust valves from rapid cooling.
In summary, run the glass packs if you like and do not worry.
Regards Marti