Depends on the type of fishing you're doing. If you're single-handing on a sprawling steelhead or salmon stream with a long belly floater, you need to change the angle of presentation with false casts before you can put the fly down. Of course this is one of the major advantages of a Spey rod, and why the fly spends so much more time in the water with two-hands, but that's another story. If the rod is overloaded, you lose roll casting, delicate presentation, long mends, and long aerialized casts like the type used on big flowing rivers. The only reason the line is even stripped is to make another cast without breaking the rod in half, it's not part of the presentation.
On the other hand if you are throwing a herring pattern into the sloshing waves straight ahead and stripping the fly down to the nail knot, overloading is a lot more appropriate. You want something that gets back to the operating length (grains) quickly.
For those in either scenario who use Teeny T series lines, the overload factor is clear - the 300 overloads an 8wt, the 400 overloads a 10wt, etc. Some people use a 400 on a 8wt, but those people have also heard of (or experienced) a sickening splintering sound occasionally.
On one end of the spectrum is the Spey double taper, never getting so much as a strip between 100 foot casts - not wanting a grain over it's ideal weight and allowing cross-river mends to skate a dry..... and on the other end of the spectrum there is the broomstick rod throwing an 800 grain deep sinking head with a 12" herring fly into a briny rip chuck-n-duck style. Somewhere in between is a nice range of compatible grains, distributions, and rods to energize them.
I have found that in both steelheading and striper fishing that I need two lines:
- the well-matched finesse line
- the get-to-business sinking or sinktip line
a) clear intermediate WF
b) hi-density sinking head with intermediate running line
Steelhead: (single hand, Spey has it's own line designs)
a) DT floating or long belly floater (70' or more of head)
b) hybrid exchangable hi-density sinktips mated with half of a WF floating head section
In both cases the (b) line overloads the rod rating where (a) does not.
Your results may vary,