Converting a CB radio from the factory-original bipolar transistor mainly accomplishes two things that I can see.
1) Slightly more power, but never enough to discern on the S-meter at the other end.
2) A much higher sensitivity to fail from high SWR, whether from a poorly-built/installed antenna or from coax-connector trouble or lack of proper ground at the antenna's mount point. The bright side of having a MOSFET final fail from a modestly-high SWR is that it's a lot cheaper to buy the replacement. Problems arise after the third or fourth time, when the foil pads begin to lift from the circuit board. The advantage of leaving the factory-stock bipolar RF transistor in the radio is that it tends to last longer.
One incidental bonus is the higher power consumption. The switchmode MOSFET transistors like ERF2030, IRF520, FQP13N10 consume more power from your DC supply to get the same RF wattage. Shortens the life of modulator transistors, modulation transformers/audio chips and base-station power supplies.
The older bipolar transistors built to use as RF amplifiers like the 2SC2166,2078,2312,1969 were designed to tolerate some SWR. Pushing them past the 'stock' rating reduces this a little, but at least they're designed with this in mind.
The switchmode MOSFETs were not. Engineers at International Rectifier were quite surprised to find that these parts would even work as RF amplifiers. They started getting calls asking for the specs you use for this type of circuit ten or fifteen years ago, and didn't believe that a IRF520 would do this at all. Apparently they had not tried it.
And if you make a living replacing blown finals, the switchmode MOSFETs are the best thing since sliced bread.
Just resist the temptation to increase the size of the radio's fuse beyond reason when it starts to trip the fuse you're using.
That's what this guy did.
Oh, and leave the "Top Gun" switch "OFF" if the radio has one.
73