I haven't used a Componon-S 240mm lens, but I have used both 240mm and 300mm El Nikkor lenses with 8x10 and a 300mm Rodenstock Rodagon (I have also tried a 240mm Rodenstock Apo-Geronar, this is a process lens that Besler sold with their own label as an 8x10 enlarging lens--it's very nice for 5x7, but doesn't really cover 8x10). The two 300mm lenses seem pretty equivalent to me in sharpness. The Nikkor is nicer to use, but is too big for my current 8x10 enlarger, it easily fit on an old Elwood I used to own.

When I got the 240mm El Nikkor lens, I did some comparison prints between the two El Nikkor. I did one comparison at a 30x40 print size (actually just some 8x10 paper in the center and corner of the image) and one smaller size (either 11x14 or 16x20, can't remember). All the differences were pretty minor and only really visible when comparing the prints side by side. In the center, the 240mm lens was slightly sharper at both maginifications. In the corner, the 240mm lens was sharper in the smaller print and the 300mm lens was sharper in the larger print. I took this to mean that the 240mm lens was a slightly sharper lens overall but had a smaller image circle. For smaller prints you have more bellows extension so you need less coverage. For big prints, you need more coverage. Since getting the 240mm, I haven't used the 300mm, but rarely print larger than 16x20. If I were doing a 30x40 print, I'd still probably use the 240mm since the working distance is much more convient and the sharpness difference is very small.

In theory, the 300mm should have less light fall-off, but I have found that this is more of a function of the size of the light source. I can't tell the difference with my two lenses with either 8x10 enlarger I've used (a De Vere with a color head or an Elwood with a 12x12 cold light). If the light source is barely large enough, using a longer lens will help with corner illumination. If the light source is oversized, a shorter lens works fine. For example, I've had some light falloff when using a 135mm lens for 4x5 with some 4x5 enlargers I've used, but using the same 135mm lens in an 8x10 enlarger when printing 4x5 works just fine, no noticable falloff. I wouldn't want to use a 240mm lens with an 8"x10" coldlight head, but with a 12"x12" head it works just fine.

Bottom line, I would imagine that the 240mmm Componon-S would work just fine for 8x10. It is certanly worth trying. If it proves inadaquate, I could probably be convinced to sell my 300mm Rodagon (I no longer own the 300mm El Nikkor).