I left my 690MBs for awhile, went to CBs, even went to Cally X18Pros, all in a new idea I was trying after I got fitted with my D1 Driver (Which I love, for now). I thought that more foregiveness in every part of my game would make a difference, and it did in my driving. But not because of the clubs, because I stopped trying to play a 40yd draw around a dogleg, I just go over it now.. but in the end I hit more fairways, but was further from the stick on my approaches. Because I do controll all my iron shots, and the MBs do it best for me. Lets face it in 8-pw, there is little feel differance, if any in a MB vs CB, but the 5 iron and 6 iron I could really tell, maybe because I tend to change the flight of the ball alot. I play in lots of wind so keeping it down, or letting it ride, can really make an impact.
So bottom line, if you want to get better, use the feedback you get to do so, but use a players club, not a PI club to do it. If you play once a month then get some PIs and hit straight (hopefully) and high (defiently)... MBs vs CBs..... you have to try to decide... what other people think or say really has no true baring, cosmetics and setup really effect some others not so much.... The real question to consider is players clubs vs player improvement clubs, and how much imporvement you want, vs fixing that swing flaw that we all have.