I own both of the speakers you are considering, the Sony SS-CS5 and the Elac debut 6.2.
I know this isn't an exact answer to your question because I don't have GR-upgrade kits in either set of speakers. However, having what I would consider adequate experience with many types of crossover upgrades and redesigns. I don't see any way that the Sony can reach the same upper potential as the Elac. Yes, the Sony is worthy of its praises at the $100 mark if that's your budget, but the extra money spent for the Elac pair earns you what I think most would agree is undeniably a higher performance and capacity. The Elac 6.2 do have something uniquely special about them at a great entry price.
Also, you should consider that the Sony upgrade kit will be significantly more challenging to assemble and install due to the higher number of parts, and more cramped interior.
Others may feel different, but I would not recommend buying the Sony for the purpose of upgrading. I support the idea of upgrading an existing pair, but if you haven't purchased them yet, I think the Elac are clearly the better choice.
In comparison, I do like the Elac's enough to recommend buying the Elac's as a new purchase for upgrading, assuming you are getting them at a good price (their price fluctuates a lot)
I've helped several others with creating layouts for the upgraded GR crossovers for those speaker sets. After making your decision and purchasing your kit, feel free to reach out if you would like any assistance.
Good luck with your decision and upgrades.
Elon
Ezeecrossovers@gmail.com