Depending on the IOPS rate for your app; SPDK can result in less CPU time spent in issuing IO/reaping completions compared to ex. io_uring.

See Ex. https://www.vldb.org/pvldb/vol16/p2090-haas.pdf What Modern NVMe Storage Can Do, And How To Exploit It: High-Performance I/O for High-Performance Storage Engines

for actual data on this.

OFC, If your block size is large enough and/or your design is batching enough etc. that you already don't spend much time in issuing IO/reaping completion then as you say, SPDK will not provide much of a gain.