The final regulations outlining minimum gateway contributions for new comparability plans
indicate that NHCEs must receive an allocation equal to the lesser of 5% or 1/3 the rate of the highest HCE. However for
combination DB/DC plans, the minimum gateway is increased by 1% for each 5% increment the HCE rate exceeds 25%.
When these final regulations were issued, EGTRRA was not yet enacted so that the allocation rate in a DC only plan would
never exceed 25%. However, since EGTRRA, it is possible in a DC only plan arrangement for an HCE to receive in excess of
25%. Have you seen anything to indicate the increase in gateway minimum, as outlined for DB/DC combination plans, now also
applies to DC only plans?

actually, the combo db/dc plans under the final regs was capped at 7.5%
1.401(a)(4)-9(B)(2)(v)(D)(2)

in a DC plan there was no change to the 1/3 or 5% rule.
Remember, an individual might be able to get 100% of pay, but will still be capped at $40,000. for ee making $200,000 this is only 20% of pay. Good grief if the ee only makes 100,000 you are only at 40% of pay, so I wouldn't worry about the fact an ee could get 100% of pay.
By the way, at 8.5% interest, 1.085^20 = 5.11,
so if your hce was 55, a 35 year old (20 year difference) would be in his rate group with a contribution of 40 / 5.11 = 7.82%.
And don't forget, the gateway is only opening the 'gate', now you have to pass non discrim, so depending on the ages you could easily end up over 5% anyway in that scenario.