If you have the resources to easily fabricate that transition then, by all means, do it. That would be very impressive. It will certainly have less loss than the plenum even if the difference is too small to measure. Bragging rights will be huge. Most importantly, to me, is if you post some pics when it's done I'll be able to link to them from other forums to show people what to strive for.

As short as the piece is sidewall frictional losses can be ignored. Momentum changes dominate the pressure loss.

Again, the differences here are small. Entrance hoods, duct length, bends, and the fan itself have much larger losses. For perspective, remember that pressure drop in nearly proportional the the square of flow rate. So the sensitivity of flow to pressure difference is about -.5. The pressure difference is referred to the effective internal fan pressure which is close to the maximum static fan pressure a zero flow. Let's say that with a lot of tweaking of something you manage to get a loss improvement of 0.15" and your fan has a maximum static pressure of 8". The, roughly, the improvement in flow would be about 0.5 x 0.15 / 8 = .009 or about 1%.