Hopefully an accountant will chip in, but... I feel like that's a little odd.
For comparison, let's say I make roughly $160k in a single owner LLC.
I pay Self-employment tax on ALL of that.
I choose a Solo 401(k) instead of a SEP IRA. I cannot contribute to a separate IRA. I am able to put $18,500 in as Employee Contributions, and 20% of the net profits in as Profit Share. $160k - S.E. tax ($24,480) * 25% = $33,880. The combined total allowed is $55k, so I can put all of that in for a total of $52,380.
After the above deductions (plus health insurance premiums) I might only pay income tax on ~$85-90k.
What I declare as my salary vs profit is irrelevant because me and my LLC are one and the same. While I file a schedule E to get business deductions, in the end, the net business income is my taxable individual income.