I've done some work in comparing pre-tax and post-tax savings on retirement income. The math is straightforward, the devil is in the assumptions, but the effect is dramatic.
Assume a 50 yr old HCE can defer only $14K instead of $24K due to the ADP limit. Assume 6% net of expense investment return, 50%/35% combined pre/post-retirement tax rates. Ignore attributable match, the ACA 3.8% investment tax, any cap gains tax rate or timing preferences. Assume equal investment expenses either in or out of the plan.
$10K for 15 yrs grows to $232,760 at 65. This may be withdrawn @ $18,208 for 25 years, pay income tax each year on the withdrawal and you're left with $11,835 cash flow.
If the $10K is refunded or not deferred each year, $5K is available to invest and the earnings are taxed each year. This will grow to $92,995 at 65. Withdraw $5,890 per year for 25 years. No further tax to pay as it is after-tax savings, so $5,890 of cash flow.
The ADP limit costs this participant $5,945 per year in retirement cash flow from age 65-90.
Even if you make the pre/post tax rates the same, say 50%, the pre-tax savings produces over $3,200 net after-tax cash flow.