I performed this replacement on my iMac11,3 2010 27" A1312, and "upgraded" to a X3470.
The results were unexpectedly POOR. In fact, the machine operated SLOWER than with the i3-550. After experiencing this, it seemed that the appropriate upgrade path was the recommended one. I bought the i7-870 chip and immediately saw the performance gain I had hoped for.
Here are my Geekbench 4 metrics for an iMac11,3 Mid-2010 27", running macOS 10.13.6, with 16Gb RAM on a 1Tb Samsung 850 EVO SSD.
i3-550: Single-core: 2419, Multi-core 5016
x3470: Single-core: 1238, Multi-core 4180
i7-870: Single-core: 2893, Multi-core 9324
While the specs suggest the x3470 to be superior silicon, my experience with a simple 1 for 1 for 1 swap demonstrated a significant reduction in performance. The Single-core index helps explain why my iMac became sluggish, but doesn't explain why things slowed. I bought the i7-870 on eBay for ~ $25... and fully appreciate the maximum performance this older machine can still deliver.