The answer to Younglearner's question is "Yes".
In the days when engine speeds were lower and most competition engines were only 2-valves per cylinder, the prevailing idea was to fit valves as large as possible for best breathing. This enforced large included angles between the valves, usually in the range of 60 to 100 degrees, which in turn forced the design of separated OHC cam boxes or towers. Once you have large spacing between the camshafts, and only have to fit in a spark plug, there can be just enough room to squeeze in the intake tract. So assuming you have a conventional in-line engine, you can have vertical intake tracts. You also have an almost straight shot at the intake port, with the added benefit that induction is aided by gravity (most beneficial for naturally aspirated engines), and the incoming charge is directed away from the hot exhaust valve thus minimizing density loss from that heat source.
There have been performance oriented engines in the past with this architecture, for instance the pre-war 2.0 Litre BMW engine, and the post-war Bristol derivative. Also, the straight 8-cylinder 2.5 L desmodromic Mercedes engine in the 1954 racing cars had this feature too, and I am sure there must have been others.
However, as Bryan points out above, horizontally opposed engines have different design constraints. For packaging reasons, it is usual to try to minimise the engine width as much as possible, and so such engines are usually short stroke, and accessories are arranged so that the cam covers are the widest dimension. This being the case, there are two things that mitigate against the adoption of "downdraught" intakes of the type discussed above: The first being simply that now the engine width is increased significantly on an engine that is already wide, and secondly with the cylinder now horizontal, the conventional "side" intake now becomes the direct equivalent of your downdraught port with all it's theoretical advantages, and your downdraught architecture is now the less favourable horizontal intake.
All in all, one would think that you would have to be crazy to design a high performance boxer engine with the porting arrangement in your question, unless you had no constraint on engine width, or perhaps if it were pressure charged. But as it happens, there have been racing engines with this design. The perfect example that comes to mind is the 1966 BRM H-16 engine, but in this case the architecture was forced upon them since the upper bank got in the way of the preferred porting for the lower bank. Thus both banks on both sides had the "between-the-cams" porting. A picture of this engine can be found at: -
However, an altogether much better review of this engine (and 49 other racing engines from 1913 to 1994) is to be found in the Karl Ludvigsen book "Classic Racing Engines", Haynes Publishing, 2001; ISBN 1 85960 649 0. I highly recommend this book to anyone who is interested in engine design, and particularly historic competition engines.
I think it unlikely that any modern engines would adopt this between-the-cams porting for two reasons: 1) Since the Gurney-Weslake V12 engine of 1967, racing engines have had much narrower valve included angles (typically around 30¡) than in the past, for more compact chambers and better combustion efficiency, so that there is no room for downdraught porting. 2) With port runners and a plenum sitting above the cambox on a conventional in-line engine, it makes for a very tall package.
PJGD
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