Guide to the Macintosh Family Hardware, Second Edition has more extensive power supply specs for the SE/30 on pages 259-260 that relate to this discussion. In particular on page 259 there is a peak load specification for disk seek servo current on the +12 V disk (4A, with certain duration and duty cycle limits and an allowed sag to 11V) and on page 260 there is what I consider a very tight requirement on +12 V sweep: The way I read it is that the + 12 V sweep supply volts will not vary by more than +_10 mV in response to any one load step on +5 V, +12 V disk, or -12 V, while supply rails other than the one being step load tested are at max load, and line input AC volts are anywhere within the wide AC spec range 85/135 or 170/270 VAC. The individual load steps are not full size between each rail's min/max current, but each designated load current step is considerable.
I speculate that the designers did not want the raster to shrink or move to any visible degree when disk seek activity or future expansion cards demanded peak current. Load step tests should be done with special attention to raster visual stability of position. It may be that commercial supplies cannot meet the strict 0.010 V sweep rail requirement, but that the raster visual appearance is satisfactory for your purposes.
Some cautions:
1. As detailed as pages 259/260 are concerning output limits, they nontheless fall far short of a design spec and/or requirements document. In particular they do not address in what order the rails decay when AC power is removed. It may not matter, but sometimes the magic smoke gets out when most rails rapidly go to 0 volts but one rail hangs in there extra long.
2. It's always safer to do some bench load testing with switched resistor loads or an active load instrument by applying a series of increasing size step load steps while monitoring response with a scope, before connecting your treasure and finding out that at minimum load the supply goes out of regulation or oscillates and threatens to smoke stuff.