The SP1-A can be made to work fantastically well. I have used them off and on since 1991 when I first got to play around with an AR3500. My mentor owned a few of them and that is the radio that made it possible for the RCI2950 to come to market years latter. The problem I think with a lot of users today is that they are into HIFI and eSSB and for the most part 99% of compression devices kill the hi fidelity and audio bandwidth. You could make a hi fidelity compression device but you have to add a lot more complexity to it. So they are just not that popular today. If you can ignore that though it is a really good way to increase average talk power to the point that your approaching the same amount of improvement you would get with a 100w external amplifier. Combine the two (100-200W amp) and compression or band pass rf clipping and some compression it is in practical terms a lot like having 500-600 watts.
I have some devices that do audio compression alone some that use all pass network and they convert the audio to rf before filtering, clipping, compression and converting it back to audio. and some that have 8 band eq+compression. All of them can sound decent to fantastic on AM and kick butt on SSB. On AM often less is more!
A Ten-Tec 715 can sound really good better than a lot of radio's built in processing.
None of this stuff is idiot approved you can mess anything up if you do not know what your doing.
You can still buy circuits based on the early industry Symetra System from early broadcast days. If you feed your audio through that alone you can improve effective talk power. Feeding it through that before feeding into a compressor also works great. AKA Phase Rotating Asymmetry Eliminator!
http://www.w3am.com/8poleapf.html
...runna pile-up on 6m SSB