Your next upgrade should really,
really be exhaust, Cow. A step up to 4" would make a
huge difference, with the only potential drawback being more noise--and that might not even happen, since smaller pipe exacerbates the obnoxious high-frequency sounds--much like a glasspack does.
Every single maxim about exhaust you've learned over the years in the N/A world gets thrown out once you've got a turbocharger. The turbine is your exhaust restriction, period, and anything restrictive beyond it only hurts performance. There is no such thing as scavenging downstream of the turbine, so the "smaller pipe makes more torque" maxim is 100% wrong with turbocharging. Necking down the exhaust at any point is a detriment. Amazingly, there are still people who parrot the "you need a little backpressure to spool the turbo" wives' tale, which is so wrong it's actually laughable. If you know anyone that tells people that, slap them. Hard. Then stop listening to them.
More theory: The turbine side of a turbocharger is not a smooth progression of exhaust flow. It's a traffic jam followed by an open freeway. The turbine wheel is driven by a differential in exhaust
pressure rather than exhaust flow itself. I'm going to shorthand "pressure differential" with the term "delta-P" to save myself some keystrokes. Remember the old adage, "Nature hates a vacuum" or "Nature hates a void"? That's because when two areas of different pressures are opened to each other, the pressure must equalize between the two. The higher the delta-P, the faster this will happen. As an example, fill a balloon with air then release the stem. It'll fart its way around the room. Knock the valve off a full CO2 tank, though, and it will become a deadly projectile. The delta-P between the CO2 tank and normal atmospheric is dramatically higher, which generates a far more violent reaction. Now stick a fan blade after both the balloon stem and the open end of the CO2 bottle. Which blade
accelerates more quickly? That's your spool time.
The greater the delta-P across the turbine wheel, the quicker it will accelerate. There are only two ways to increase your delta-P: Either increase your exhaust drive pressure (between the combustion chamber and turbine) or reduce the pressure in the exhaust system downstream of the turbine. Increasing drive pressure is a case of seriously-diminishing returns, since when the exhaust valve is open, the spent gases will have a harder time leaving the area. That means less room for fresh, more-combustible air in the chamber. The more-effective way of increasing delta-P is to reduce pressure in the exhaust system. Larger, smoother (mandrel-bent) pipes and less-restrictive muffler(s) will make a noticeable difference in spool time, meaning you'll start to generate boost at a lower RPM. There is literally no detrimental effect anywhere else in the RPM range, since a higher delta-P always means greater turbine speed. The turbine has no idea how fast the engine is spinning.
It's been said that the best turbo exhaust system is no exhaust at all. That's not quite true, since due to turbulent airflow the best exhaust system is actually a short megaphone, 6-8" long, with the sides of the megaphone angled 10-15° off the centerline of the exhaust path (20-30° included angle between the megaphone sides). However, testing has shown that the megaphone still makes a measurable difference when used as a transition to a full exhaust system. Obviously, space considerations are going to limit what one can do as far as following the guideline, but the most-important takeaway from the megaphone concept is that you need a smooth, gentle transition to the larger pipe coming off your turbine's exhaust outlet. I've seen a lot of guys, some who should know better, just install a huge step transition right at the turbo. That sharp step generates turbulence, and turbulence impedes flow. That's also why you'll never see integral wastegates on max-effort turbocharged anything; an integral wastegate's sharp flow angle back into the exhaust stream creates turbulence right at the turbine outlet, which hurts flow and hence delta-P. A somewhat-common practice is to divide (or "divorce") the wastegate exit area on the turbine housing, and create a flow path for its exhaust separate from the turbine outlet, with the extra pipe re-entering the main exhaust pipe at as shallow an angle as possible, as far away from the turbine housing as space will allow:
