Yes, the carbine port position erodes faster than a rifle port. HOWEVER, the difference is moot - the port in a carbine lasts longer reliably than the barrel's effective service life - meaning you've burned out the throat and "ringed" the barrel in most cases before the port erosion ever starts showing degradation issues.
Port erosion problems really only show up in rifles which are shot long past the appropriate service life of their barrel - most of us who actually want to hit something will replace our barrels when a 3/4MOA rifle opens up to 1-1.5MOA, or a 1MOA rifle opens up to 2MOA, predominantly caused by throat erosion. Guys who just turn money into noise as fast as they can and never worry about their rifle hitting anything will burn their ports up too, but they won't really notice the effect since they don't actually hit anything anyway, so they won't notice any change when it doesn't hit anything... but worse...
A Ford F-350 diesel will last more miles, have more power, and get better fuel mileage pulling a heavy load than an F-150. Doesn't mean the F-150 isn't one of the best trucks on the road for its purpose (blended light duty hauling as well as no-load driving with a lower capital outlay), so it's a better suited truck than the F-350 for most consumers. The long barrel AR's might last longer than short gas AR's, but at the end of the day, getting 280k, 440k, 320k, 240k miles out of F-150's before I sold them off (the 440k did seize up) is more than I ever needed to get out of them. Getting 40,000rnds out of a Bushmaster carbine (6 barrels) has absolutely been enough "miles" in my book to quantify that as "beyond acceptable service life."
The major effect of gas port erosion is an increase in cyclic rate.
A straight walled tube is not a good nozzle. The actual constriction is smaller than the diameter.
If you look closely at the velocity maps I posted earlier, you can see the vena-contracta jet.
Most people believe before erosion can cause a problem, it has to wear to the point where the gas port is actually a larger diameter. This is not true.
If you look at the above image, you can see the "coefficient of discharge", or simply the efficiency of the orifice in passing mass flow, increases if you round the leading corners of the orifice. Note that the actual hole through the material is still the same size. If you look at the image below, you will see that if the leading edges are rounded to a certain point, the diameter of the vena-contracta will be able to expand to the full diameter of the orifice.
Gas port erosion wears the barrel bore side of the gas port to have rounded corners, and this makes it a more efficient nozzle. Thus, more mass can flow through the port, even though if one were to measure the port from the outside with a pin gauge, it is still the same diameter as it started.
In as few as 2500 rounds you can see a cyclic rate increase of 100 rpm, it will increase until the bore side of the port has worn to the most efficient shape, and then the cyclic rate will stabilize, around 3500 to 5000 rounds, depending on the barrel material, hardness, pressure, etc, etc.
Unless you start out with a overly high cyclic rate, this really isn't much of a problem. As a point of reference, the Army notes that if the cyclic rate gets too much over 1100 rpm, M4s start to break parts at an increased rate.
