Based on my experiences with my Rel Stadium 2 I will definitely say the answer to your question, unfortunately, is "YES", with utmost certainty. I say "unfortunately" because it's the proverbial double-edged sword. On the one hand, if it didn't make a difference you'd be saved the extra costs of both...while the good news is that it lends 2 additional avenues of audible system improvements through cable upgrades.
As for pc's, my Rel went from stock pc (the unit "functioned correctly" but only at perhaps 60% of its ultimate potential)...then onto a Signal Cable Magic Power which was a nice improvement (I'd rate it up to a solid 75%), improvements included a noticeable speed and agility increase as well as better bottom extension...).
Then I tried a Pangea AC9 which was a big disappointment...even after a lengthy break in it was a step backward from the Sig Cable and only "somewhat better" than stock..let's say it now went back to a 67%...
Then, a significant and thoroughly justified improvement when I switched to a VH Audio Flavor 4: MUCH faster, deeper, and much improved sense of power and authority... the Rel was finally showing its attributes and justifying itself in the system (also in my experiences a good sub not only disappears better but also adds a more natural presence, focus, imaging and depth to the overall soundstage that is sorely missed when not there, even with speakers you'd never consider lacking in the bass department). I would say the Flavor 4 pushed the Rel up to its 90+% of what I would imagine its potential to be. There must be room for more improvement but at this point the performance/cost ratio is above my wallet at the moment.
As for I/C's I had been using the supplied Speakon connector at first but had Frank at Signal Cable build me a custom one to take its place which I happily used for several years (also an improvement over the original, similar to the powercords... as it was better than the stock-to-Pangea but not quite the Magic-to-Flavor 4 leap). When I had to change rooms I also changed my Rel set-up and went with I/C's. Here I used "spare" Harmonic Technology Pro Silway mk 2's which did a fine job and stayed as my default for a long time. Finally, and just recently I had a pair of Harmonic Tech Magic 1 I/C's available and swapped them in place of the Pro Sil 2's ...not knowing what to expect, and lo and behold a significant jump again.... much more so than expected and even more dramatic than the Magic power to Flav 4 PC upgrade. This was a clear improvement in air and space, an even better sense of effortlessness, speed and "tightness" with the most seamless integration into the overall soundscape ever.
I know this is blasphemous but part of me had secretly hoped there wouldn't be much, if any difference so that I could sell the pricier Magic rather than the pro sil 2, but the difference was not only there...it was instantly appreciated to the extent that I couldn't go back again.
Anyway, I hope I didn't bore you with the specifics but sometimes I find passing on personal experiences help in someone's decision making processes.
IMHO, I would start with some kind of decent aftermarket powercord just to dump the stock cable ( there are a lot out there and a new system favorite recently is a company called TWL (Triode Wire Labs) which I'm having good luck with elsewhere including amps, pre, and digital...) as well as the aforementioned Signal Cable, etc.
I/C's you can explore on your own, as I'm sure you have favorites already worth trying, but the key here is: DO TRY!
Good luck and Happy Lissn'n