| Admiral, on the He 280, I think the main reason for the relatively low Mach limit was a result of the tail, with controlls locking up at high speeds iirc. The company's figure for limiting Mach number was .79 according to Delcyros.
There were also some structural problems with the tail (similar to the He 162's?) and it needed to be redesigned to be realy ready for production/combat. I believe a single fin "normal" type tail was suggested for both improving the faults and for easier production. (not sure on that)
However, like most contemporary German jet projects it lacked air brakes. (the Ho IX/229 had brakes though, the He 280, He 162, and Me 262 did not, I don't think the Ar 234 did either)
And on the engines, anything larger than the 003 would be unsuitable, though the airframe could accept the 004 and was tested with them, the range and maneuverability was drastically reduced compared to the original HeS-8 engines, and top speed was about equal to the best obtained with the HeS-8's. The 003 was a decent fit but available a bit late.
The HeS-30 (006) was very goos and about the same weight as the HeS-8 while being only 24 in in diameter and having excelent fuel consumption and thrust output similar to the 004. Due to being cancelled it's hard to tell how soon it could have been in production, but from what I've read, likely around the same time as the 004B. And it obviously would have been very advantageous to all the jet projects' performances.
As the HeS-8 is more well documented and was reliably producing 600+ kp (1,321+ lbf) thrust in realistic flight conditions (~674 kp; ~1485 lbf in testing) before being canceled in 1942 I think it would have been good enough and available earlier than the others if produced as soon as all the major bugs had been worked out. (which it pretty much was at the time of cancelation, albeit well short of the planned thrust) Performance was acceptable and it was well matched to the He 280 (being basicly designed for the a/c). And it could probably have been deployed practically on the He 280 in the 1943 timeframe.
As I've mentioned before, it may also have been a good idea to modify the the He 280's wings in a similar manner as the Meteor was designed (with curved spar going aroung mid mounted engine) to allow the larger (~39 in diameter) HeS 6 engines to be accepted into the airframe. It would have added weight, but the HeS 6 engines were working better early on than the later HeS 8 (being developed along the HeS-3b development at the end of 1939) producing about 1,300 lbf (590 kp), however it was abandoned due to the large diameter and fairly high weight 925 lbs (420 kg), but seing the timeframe, development, and thrust output it may have been a good idea considering the later problems (at leas as an intrim measure), and the HeS-8 wasn't producing similar thrust for another 2 years.
And who knows how performance would have improved with continued development and with Hirth's help. | 
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