Fissionable armor=Increased nuke yield?

U-238 could not possibly have this effect, and for any fusion-boosted warhead with a high enough pressure and/or neutron to flux to achieve criticality in U-235 armor, the contribution from the armor would be extremely small.

Nuclear warheads are touchy things. It's really hard to make it work right.

That's not how tampers work. The act a bit as neutron mirrors but mostly the tamper is provided mechanical containment to prolong the principle reaction.

"Uranium-238 and thorium-232 (and some other fissionable materials) cannot maintain a self-sustaining fission explosion, but these isotopes can be made to fission by an externally maintained supply of fast neutrons from fission or fusion reactions. Thus, the yield of a nuclear weapon can be increased by surrounding the device with uranium-238, in the form of either natural or depleted uranium, or with thorium-232, in the form of natural thorium. This approach is particularly advantageous in a thermonuclear weapon in which uranium-238 or thorium-232 in the outer shell of the secondary capsule is used to absorb an abundance of fast neutrons from fusion reactions produced within the secondary. The explosive yields of some weapon designs have been further increased by the substitution of highly enriched uranium-235 for uranium-238 in the secondary."

"All so-called "fission-fusion-fission" weapons (i.e., all conventional modern thermonuclear warheads) employ the additional step of "jacket fissioning," using fusion neutrons. This works as follows: the high-energy or "fast" neutrons generated by fusion are used to fission a fissionable jacket located around the fusion stage. In the past this jacket was often made of natural or depleted uranium; but today's weapons in which there is a premium on weight and size (i.e., virtually all modern strategic weapons) use moderately-to-highly enriched uranium as the jacketing material. The fast fission of the secondary jacket in fission-fusion-fission bombs is sometimes referred to as a "third stage" in the bomb, but it should not be confused with the obsolete true three-stage thermonuclear design, in which there existed another complete tertiary fusion stage."

Back on topic, though... does the game actually recognize this, or does the use of a high-density refractory metal like uranium impede the effectiveness of the bomb?

Actually, both are correct, as they are used to provide a uniform compression to whatever lies inside, whether it be the fissile "pit" of the primary, or the fusion fuel for the secondary. In the latter case, it wraps around the fusion fuel (usually lithium deuteride if memory serves) and essentially acts as both insulation from the plasma and high intensity radiation. The tamper actually ablates off a part of its mass rather quickly and violently, and this creates the compressional shock that primes the secondary for ignition by the fissile "sparkplug" at its heart: