Your point is pretty moot when the engine is that same & as far as I can see any efficiencies drawn from one title (DMC4) using the same engine on the same platform can easily be brought into another (LP) game using the same engine on the same platform (PS3).. Regardless of whether the game is finished or not..
Lost Planet's content was designed for the Xbox 360.
Lost Planet for the PS3 is a classic "port" and not co-developed on multiple platforms.
The game is nearly a year old and up against robust competition. The port is there to maximize return on the title--the content is finished and the engine has been ported to the PS3. A
port is relatively trivial. We hear all the time how art assets, and not the engine, are the most expensive part of development.
The memory resource disparity between the two consoles is well covered (less available RAM on the PS3 due to the OS and framebuffer, split pools requiring more management). Even if the difference is 50MB, if 250MB of the 360's memory is for textures and geometry that would translate to 20% less memory for such on the PS3. So does Capcom rework all the art assets for a title that may only sell a couple hundred thousands units (1 year old ports don't always sell great), or do they apply more aggressive LOD and remove some of the higher resolution details?
The game is also fillrate intensive, hence why the 360 version uses a quarter sized buffer for particles. Regardless of the engine the PS3 will be even more taxed unless they put a bit of work into the title work around this.
Capcom isn't going to redesign LP for the PS3. It is a port, and old one at that. The idea that DMC4 looks great so LP must as well ignores how LP used the 360 and the nature of the product. If Capcom spent the time and money to rework some of the balance and art to be more mutual between the two I can see your point. But "as is" the game, not the engine, was designed for the 360.