I'm not familiar with what Ethereum requires for its syncing operation.
If it's a sequential write (by downloading the entire blockchain), you will still be bottlenecked by the throughput of the underlying disk.
If it's sequential reads (in between writes), the reads can be handled by the cache if the location is local enough to the previous write operation that it hasn't been evicted yet.
If it's random unpredictable reads, it's unlikely a cache will help unless the cache is big enough to fit the entire working dataset (otherwise you'll get a terrible cache hit rate as most of what you need would've been evicted by then) but then you're back at your original problem of needing a huge SSD.
If it's a sequential write (by downloading the entire blockchain), you will still be bottlenecked by the throughput of the underlying disk.
If it's sequential reads (in between writes), the reads can be handled by the cache if the location is local enough to the previous write operation that it hasn't been evicted yet.
If it's random unpredictable reads, it's unlikely a cache will help unless the cache is big enough to fit the entire working dataset (otherwise you'll get a terrible cache hit rate as most of what you need would've been evicted by then) but then you're back at your original problem of needing a huge SSD.