Dear Daniel,
Anytime you introduce a tele converter with a lens, the optical performance is almost always h&ered to a certain extent. The Schneider 1.4 Longar is optically calibrated for use with the 180 2.8 Schneider Tele Xenar as well as the 300 4.0 Tele Xenar and the 80 2.0 Xenotar lenses. The 2.0 converter is more of a universal type converter, and the optical performance with the 180 2.8 will not be on par as compared to the 1.4 Longar.
There are two versions of the 180 2.8 lens with different MTF ratings. If the rear bayonet of your 180 2.8 lens is st&ed CE, than you probably have one of the newer version optical formulas. A serial number of # 14581933 or prior generally fell into the catagory of the older optical formula version of the 180 2.8 Tele Xenar lens. The older version actually has a higher resolving power from the center of the negative close to the outer edge, but the newer version has better overall performance from the center to the very edge of the negative. Nevertheless, both versions are stellar performers of a very high order.
My experience with using the 1.4 Longar with the 180 2.8 Schneider is that the shawdow detail and edge sharpness is not quite as good with a size 16"x16" print. Depending on your subject, you may or may not notice a difference. With landscapes, I doubt that you would be able to see any difference at all. If your subject contains very fine and intricate detail, and you are able to view two prints side by side of the same subject, one taken with the 1.4 Longar and one without, then you might notice a difference. Moreover, shooting at 2.8 with or without the use of the 1.4 Longar, I cannot discern any difference in the optical performance other than the difference in the focal length perspective.
Regards,
Dale