Other than strength of the signal itself, atmospheric vagarities, and attenuation of the signal after the LNB, reception strength depends on three things, 1) aiming, 2) how strong the amp in the LNB is, 3) how large the reflector is and how well it is designed.
1) Aiming can be trickier with the 5-LNB, but just as accurate. It depends upon the motivation and skill of the installer.
2) The amplification is probably designed so that the final output of the dish is equal to or greater than previous models. This is a factor that they have a lot of control over, and it would be problematic for them if they didn't do it this way
3) The reflector size is important, but when designers veer away from a parabolic dish (one focal point) to a parabolic-spherical dish (multiple focal points) some tradeoff in signal level is inevitable. The focal points defocus a little bit. Of course all current dishes are already parabolic-spherical, but there may be more tradeoff in one with 5 focal points. Its important to reflect enough signal to the LNB for the level to be within the window of operation for the LNB's amp, so there is a minimum size that will work. Also there is an acceptance factor. At some point the customer (wife) says "no way" to a larger dish.
Bottom line, the SlimLine will probably perform at least equally to the 3-phase.