I did a bit of testing with 2 wire antennas this weekend. Here is a summary of the results, which I also posted to the IC-7600 e-mail reflector:
The antennas:
1 – A “long” 80m Dipole, ladder line, 4:1 balun, then a long coax run
to the rig
2 – A fan dipole with 80/40/20 meter elements (these are close-spaced
elements, with 80m and 40m using opposing legs of a single piece of
300-ohm twinlead), 1:1 balun, then a long coax run to the rig
This was a brief test. I tried only 10/15/20/40/80 meters, and tested
only a few frequencies on each band, including something close to the
bottom and top of each band.
The IC-7600 internal tuner was able to tune both antennas over most of
the 10/15/20/40/80 meter bands with the following exceptions:
1 – Could not match the 80m Long Dipole at the high end of 80m or
anywhere on 40 meters
2 – Could not match the Fan Dipole anywhere on 20 meters
The External LDG-100 PROII was able to match all the test frequencies.
The IC-7600 tuner is very good, and tuned most of the frequencies,
including some that appeared to be worse than 3:1 SWR (the stated
spec) on my antenna analyzer.
Please note – I am NOT knocking the internal tuner, and with a little
antenna work (pruning antennas and feedlines – not fun) I could
probably get matches everywhere I need to operate. So, while the
internal tuner is very good, the LDG (which has a nice ICOM interface)
covers a wider range. The LDG was sometimes very slow (10s of
seconds), but once a match is found it is stored and recalled quickly
next time you need it.
Kg3v Ham Radio Blog 
Hi, How is your LDG tuner connected to the IC7600? Does it respond to the tuner call switch on the radio? I want to use an LDG IC-100 tuner with my new IC7600 for 80meters. I saw some forum posts that said it wont work.
Tnx & 73
Jeff
kb7yo
Kalama, WA
Jeff,
I use the 4-pin Icom connector. I had to buy the Icom cable for the tuner separately. It connects to the tuner, and provides the standard “AH-4 compatible” interface used on the IC-7600.
Much the same experience here – the 7600’s internal tuner would not match an 80m dipole on 40m (but is FB over all of 80m + WARC bands). The LDG AT-200Pro does just fine on 40m. 73 Joe/AC0DR
I do find that the 7600 tuner is quite fast. It is much faster than my LDG when I first visit a new frequency. Now I am trying to get my antennas within range of the 7600 due to the added convenience. I also set the 7600 to automatically start the tuner if necessary, when I first apply RF. This has greatly increased my speed moving around the Bands in contests.
73, Tom KG3V