First off, 30 psi or even 35 psi with a turbo that is capable of it is not a problem on our engines. 40 psi is really the limit you want to go with a stock head gasket.
Yes a smaller housing will give you better spool, to a point. The 14 cm housing is not a bad choice with the stock 52 mm wheel. The reason you really don't need a WG is the design of the compressor and turbine will help to limit the max psi.
Even with minimal upgrades the non-WG housings and the small compressor wheels loose their ability to stay in front of EGT's. You won't see this in a DD but drop a load on and it becomes apparent.
If you want a turbo that is capable of flowing more air and by default building more boost then you have to address the compressor wheel size. A 58 mm compressor with a good turbine is good for about 35 psi and makes pretty good air from 15 psi up to 30 psi. A 60 mm compressor in a correctly sized housing will make 38-40 psi of good air and flow more in the 18-30 range than a smaller compressor. The spool on these turbos is going to be very comparable.
What we need, have always needed, is BETTER air at lower boost numbers. You get that with a turbo that is capable of making more boost and flowing more air on the top end, or going to twins. Some times you need to upsize to to get what you want in the lower ranges.
An HE351 is 60\65\9 Holeset turbo off the 04. 5+ CR. Good upgrade
An HE341 is a 56\65\9 Holeset turbo off an 03-04 CR. Good upgrade.
An HX35 could be a 52-54\58-60\12 off of a range of trucks thru 2001. You need to check and see what the compressor and turbine size is. The early HX35 was not much of an upgrade over the H1C.
An HX40 is the turbo off of the older C series cummins. They were not a high speed quick spool turbo and are really old technology. You can do better shopping around.
An S300 is a Schweitzer turbo off of a different application.
Post up the links on Ebay and someone will take a look and offer an opinion.
FYI, my son and I have an HE351VE on 2 of our trucks. They are work to adapt but are great. The other one has a 56\60\16 non-WG turbo that works well but suffers when you load it down. We run these trucks to 35 psi on a daily basis so don't sweat boost under 40.