what i'm saying is that the degree to which it will affect the spool will be indiscernable.
how does a bigger diameter hurt spool? well, a first-order effect would be increased internal volume. however, let's look at how long the up pipe is: not very. any increase in internal volume is relatively small... a couple of cubic inches at most. certainly smaller than that increase seen in a case of switching from oem to aftermarket tube headers. i don't see the increased volume directly leading to any measurable increase in spool time or boost threshold.
what about other effects? well, having a lot of changes in diameter is not good because they create turbulence. i can tell you that on the oem RH collector, the diameter of the port leading to the up pipe is QUITE small. it would be ideal to enlarge this to match any up pipe you fit. there's a lot of meat that could be removed. however, if this is not accomplished you have essentially created an anti-reversion step of smaller diameter to larger. still not a bad thing to have, despite the fact that you have a diameter change.
the arguement that the increased internal up pipe diameter slows the stream of exhaust gas into the turbine doesn't make sense to me. assuming we're talking about spool up, where the wastegate is shut, the system is closed--any gasses exiting the heads MUST eventually pass through the exhaust housing and turbine. if you have a garden hose attached to a faucet on one end and a nozzle on the other, whatever exits the faucet MUST pass through the nozzle.
the only things that matter are:
1) the lack of internal friction (ie, restrictions, turbulence). to go back to the garden hose analogy, this would be like a kink in the hose restricting flow. notice that when the nozzle is shut, you get full water pressure at the end, because there is no flow rate. however, when you open the nozzle, the restriction creates a pressure drop proportional to flow rate.
2) the retention of heat energy. obviously this up pipe is no different than any other: we seek to retain as much heat in the exhaust as possible. wrapping and/or coating is a very good idea.
if we haven't changed the A/R or nozzle shape/dimentions in the housing, then we haven't changed how hard or how fast the gasses are going to impact the turbine. all the nozzle "knows" about is mass and pressure. again, back to the garden hose analogy: what if we have a section of hose in the middle that is of a larger diameter. the couplings to our original hose are designed to have smooth tapers to minimize friction and turbulence. will the nozzle at the end of the hose be able to squirt water with the same force and pressure? basically, yes.
so in short, do your due dillgence in fitting this pipe (as you would any other)--gasket match both ends, insulate it, and make sure there is a smooth coupling to the exhaust housing--and there is no reason to think your spool is going to be any different. in fact, unless your old up pipe had the same attention to detail paid to it, it'll probably be BETTER.