BMW 139 seems to have been similar in performance to the R-2600-8 if the latter was stuck with only the high supercharger gear. (both roughly 1250 hp at 16000 ft) Going by the chart here
http://www.ww2aircraft.net/forum/engines/2600-table-28832-2.html
http://www.ww2aircraft.net/forum/attachments/engines/166968d1304950872-2600-table-26-vs-17.jpg
and going off the figures for the 139's performance mentioned in the Fw 190 thread here:
The 139's 30 min rating seems a bit better than the R-2600-8's military rating though. (1410 ps ~1390 hp at 4500 m ~14760 ft vs what looks like 1325 hp at the same altitude) Plus the 139 was smaller in diameter and lighter.
I've seen some claims to the BMW 139 being a 'failure' but most of the more detailed explanations for mass production being passed over in favor of the follow-on 801 project was simply the technical superiority of the latter. (I believe tomo_pauk made the argument for sticking with the earlier design and pushing it into production in place of the 801, allowing an earlier introduction of a significantly lighter, simpler, yet somewhat lower performing engine)
Likewise I can't seem to find much reason for the Bramo 329 being an actual 'failure' other than BMW wanting to concentrate on their own designs. That engine seems like it had more potential to be an early-war counterpart to the R-2800 (though closer to the R-3350, Centaurus, and BMW 802 in displacement) bench testing at 2000 hp (or possible PS) in october 1938.
There also seems to be a lot of confusion with authors referring to the 139 as an 18 cylinder engine and the 329 as 14 cylinder, or both as 14 cylinder (and others showing photographs of some mysterious pre-war 18 cylinder engine), but the information I've seen points to the BMW 139 being a 14 cylinder design of 155.5 mm bore and stroke.
Given the discussion in this thread
http://www.ww2aircraft.net/forum/engines/bmw-139-information-13261.html
and the specific reference of the Bramo 329 using a 154 mm bore and stroke, even though it also lists it as 14 cylinders (and 40.2L displacement) it still seems likely that the bore/stroke figures are accurate and based on a shortened stroke version of the Fafnir's cylinders. (and thus a 51.63L engine)
It really seems like a good deal of the information on the 139 and 329 has been jumbled up over time and made a bit more confusing with the (projected but unbuilt) 18 cylinder BMW 140 design also being present prior to the 1938 merger.
It seems like the BMW merger may have done more harm than good there, or at least resulting BMW-led management. Rather than keeping the 329 in development as a larger, more powerful radial engine to complement the smaller, lighter 1500 ps take-off 129 (and indeed, potentially offer a 2000 ps class engine years before the Jumo 222 might have been ready, let alone 2000 ps class 603 or 213 models). They would have had 2 very useful bomber and transport engines potentially also useful for fighters. (the lower weight of the 139 seems like it would make a better fighter engine than the 801 did -or contemporary R-2600 due both to weight and frontal area there, especially with the cooling fan arrangement)
The 329 should have been a good match to the Do 217 while the 139 seems like it would be a better fit for the Ju 88 and possibly some He 111 derivatives (possibly the Ju 252 -though the 329 would have allowed that to get by with just 2 engines more like the C-46) among other possibilities. Aside from potentially keeping the He 111 more competitive as a bomber/patrol aircraft, there might have been more potential there in the transport role. (mostly if the fuselage was expanded or stretched, but at some point that's more a new aircraft just using the He 111's wings -something more akin to the Super Electra to Lodestar transition would merit engines more in the Jumo 211 class anyway)