Removing only two F seats on the 737-300/700 would not create enough room to add an additional row of coach seats, since the configuration is presently three rows of four seats abreast. The mod to the 737-500 was accomplished by removing the large closet and combining the 1R galley with a new, smaller closet unit. This gave enough room to add 1E/F (which previously did not exist) to offset the loss of row 3A-F. Pitch in F was reduced, and the bulkhead was removed to allow for two additional Y rows to be installed.

The problem with doing this to the 737-300/700 is that there really isn't that much wasted space in the configuration to begin with. Adding more coach seats at the expense of First probably wouldn't create enough additional revenue to justify the cost of the mod. The 737-500 reconfiguration turned dead space into revenue-earning seats, this would not necessarily be the case with a 737-300/700 mod.

Furthermore, these aircraft, especially the 737-700 (SNA anyone?) operate in a number of markets where CO enjoys a significant revenue premium driven by the sale of F tickets, so removing these seats with a higher yield potential does not seem to be a prudent move.

Don't forget that CO is also flattening its domestic ASM growth for '08 and beyond, and with a number of high-capacity 737-800/900ER ships on the way, much of this will be offset by 737-300/500 dispositions. Doesn't really make sense to invest a whole lot of money into an airplane that's on its way out of the fleet anyway, does it? (bear in mind winglet mods, at least on the -500s, are being financed by lessors, if I'm not mistaken)

The 737-300s at USAir either have 8 or 12 F seats, depending on whether your airplane is an original US or HP bird.

And to answer your question re: the 757/767 at US

-the US 757-200s have a higher seating capacity because US has Y seating between the 1 and 2 doors, where on CO ships, this area is exclusively BusinessFirst (16 seats). Even their new international 757s (the ex-TZ birds) have 3 rows of Envoy (12 seats), followed by 11 Y seats in a space that is occupied by only 4 BF seats at CO. As anyone who has flown on a CO 752 can attest, the Economy cabin is pretty densely configured, so I don't think this is an issue at CO. In fact, I'm sure CO would love to have another row or two of BF seats on the 752 for some of the premium transatlantic markets they serve out of EWR, but do not want to sacrifice 18-24 Y seats to do this.

-the US 767-200ER has only three rows of Envoy seats at six abreast, where CO has five rows of BF seats at five abreast. The space on the US 767 that would be part of the BF cabin on CO is filled with coach seats at seven abreast. CO does not want to undermine the integrity of its BF product by dramatically reducing width to accommodate 2-2-2 on the 767, so the result is a less dense configuration that preserves the width of the BF seat and allows for a sufficiently large premium cabin to meet international demand.