I've meaning to write a review of Tech-T's Hush bolt for Electra/Pilot for a while, so here's a start. Hope it helps:
I played about a year with the 09 Electra before getting the Hush bolt. Since getting it I have played 4 times, so I hope I have a good feel for it now. I bought it new from my local store for $45. Generally I like it and it is an improvement, but there are some downsides which I'll explain, the biggest of which is the price.
First off it has a nice look and is well made, everything is smooth and well finished. It has 3 o-rings which help contain the air for more efficient use, and the o-rings on the bolt also prevent that little bit of air from escaping out the back of the bolt and blowing on your face (you don't notice when you're wearing a mask anyways). It has a Venturi face, with the holes traveling about a inch towards the part of the bolt that the air enters thru. The face is cupped and the top edge is rounded, probably to try and reduce chopping/clipping.
The biggest plus is that when you put it in, it does increase your velocity significantly as advertised, approx. 30fps. This allows the output pressure of the regulator on the marker to be turned down, which does reduce the kick by a small but noticeable amount. The efficiency is better too, my tank usually has more air in it when I fill up, although I haven't counted balls fired per air, so I can't say exactly how much. If I didn't have access to air between games, this might be a bigger selling point for me.
The sound signature of the marker does change with the bolt. The actual noise changes from a sharp crack to a slightly duller thunk. The marker is slightly quieter too, but I believe its more because of the pressure change than because of shape of the bolt distorts the sound. Either way, this change isn't all that significant and you'll really have to be listening for the difference to notice it. I've observed while playing with it that my electra (with J&J barrel) is still louder than a stock SP-1 or BT TM15, and sounds pretty similar to a stock mini.
On of my gripes is that the Hush Bolt does not come with the connecting push pin, ball bearing, and screw that holds the bearing in. You'll need to remove them from the stock bolt and put them in the new. The stock screw felt loose to me in the Hush bolt, and at the end of the first day I played with this, the screw came out of the bolt during play and I lost it and the ball bearing. Kingman's support was kind enough to replace them, and the replacements felt like a much tighter fit, but I would recommend some Locktite to anyone making switch. Either way, you would have to buy a replacement screw/bearing/connecting-pin if you wanted to be able to just drop the stock one back in. This seems like something Tech-T should have included with the bolt for the price.
The other downside to this bolt that I've noticed is that even a little bit of paint on the bolt will cause the marker to not re-cock every time (probably because it has o-rings). The stock bolt seemed better able to shoot thru a little bit of paint in there. Its also worth mentioning that in the 4 days I've use the hush bolt, I had 1 chop on each of 3 of the days. Using the stock bolt I had 1, maybe 2 chops/clips over an entire year. I've tried a variety of different grades of paint thru this, and all of the chops/clips happened with low quality paint. I suspect that the Hush Bolt is just less forgiving of rec grade paint than the stock one, but obviously I can't confirm as paint quality varies so much.
To sum up, this is a nice upgrade if you are looking to lower the pressure in your marker or get better efficiency, but it has some drawbacks, and the improvements are not all that significant. Don't get it if you are just looking to quiet your marker. At the current price of $45 this is not a must-buy upgrade and is probably more trouble than its worth, but if you could get it for cheaper then it is an improvement.