O.K., get ready. This is going to be a bit long, as I think you are about to get in over your head if you are not careful.
Best BOV is a BPV, and it happens to be the one that came with the car -- stock. Must run recirc and not blow off to atmosphere. The stock valve is very sophisticated dual diaphragm design that has been tested by Grass Roots Motorsports Magazine against the most popular aftermarket valve -- the Forge. The stock valve held almost 2 psi HIGHER boost in the upper gears. It requires no adjustment, will not make strange sounds, maintains excellent drivability, holds boost to well above the capabilities of the stock K04 turbo, rarely fails (far less so than most of the aftermarket so-called "upgrades") and best of all . . . it is free. Free means more money for upgrades that will actually improve performance.
Whatever guys think about diverter valves, either BPV or BOV configuration, based on their experience with other platforms, needs to be thrown in the garbage can when it comes to this engine. Mazda did not make a perfect turbo engine, but one thing it got right, probably better than any manufacturer out there, is their stock BPV. Period.
As to the other questions:
An intake upgrade is a serious power producing mod for the MS3 and MS6. I do not know what the intake is on a CX7 and whether there are any aftermarket upgrades for it. If the configuration in the engine bay is different, our intakes might not fit. Maybe post in the CX7 section of this forum for that info. On our cars, a good aftermarket intake will produce a gain in the 15 whp/15 torque range. Serious improvement.
As to downpipe -- be careful. First, the turbo on the K04 is not exactly the same as on our cars. Also since the MS3 and MS6 use different downpipes from each other, IIRC, I have no earthly idea whether either would fit the CX7. I speak only in general terms and regarding the MS3 only from here on.
On our cars, the stock downpipe and mid pipe contain separate low flow catalytic converters. Both are highly restrictive, as is the shape of the stock downpipe. A good aftermarket downipipe either catted or catless in combo with a replacement test pipe (or race pipe) really wakes up this engine. But at a cost, and I'm not talking just dollars. An increase in flow from the improved intake and correcting the serious restrictions in the downpipe and midpipe puts a serious demand on the stock fuel pump and also changes fueling demands in ways that can be dangerous for the engine if you do not do some other upgrades at the same time.
This means that with the increased flow on the exhaust side, your stock fuel pump is likely to poop out on you. That could result in an AFR lean out and engine damage or failure. On the MS3 and MS6, upgrading the fuel pump internals becomes very important. That is about $350 in parts plus labor or DIY. (Not very difficult).
But, that's only part of the concern. Your ECU is not programmed for the increased flow. When I went with the 3 inch dp/rp in my sig below, I noticed that there were nasty 20-21 psi spikes in boost when first going wide open throttle. I think that is dangerous in the long run. You need an ECU tune to fix that. I do not know what options are available for the CX7. I've never researched that. The Hypertech tune I have works very well in fixing the spike and in making my other mods optimized for the stock K04 turbo.
With increased flow and fueling demands and changes in safe operating requirements, these mods require some type of engithe tne monitoring software or device. Simple intake upgrade works well on the MS3 and MS6 without need to monitor. Go beyond that and you really need to know your boost level and know the fuel rail pressure of the cam driven high pressure fuel pump, and need to know your AFR's, especially under load. It is desirable to know if you are getting any timing retard (knock retard) due to unsafe operating conditions. In other words, on the passenger car versions of this engine, you really need to monitor these engine parameters if you are going to be running engine mods beyond a simple SRI or CAI intake.
I wish I could be more help. It sounds like you are new to mods on our direct injection turbo engines. There are enough differences in the turbo and in ECU programming for the CX7 that I am reluctant to make any specific recommendations.
EDIT: A quick check of vendor sites does indicate that several of the major manufacturers do make aftermarket intakes and downpipes for the CX-7, particularly CP-E. They are pretty proud of their products according to cost. As to tuning or monitoring devices, the only tuning I could find is VersaTune. It is a good product that is gaining some respect with MS3 and MS6 owners, but Cobb's AP owns that market. But neither Cobb nor Hypertech offers a tuning solution for CX7 as far as I can tell. With VersaTune, you will probably need to use a good tuning company to dial in that tune to be safe. Talk to Lex over on the mazdaspeed forums board about that. He is very respected and tunes both AP and VersaTune, IIRC.
