I've recently bought some emtec 8x +R discs
but my NEC 3500A writer (with now hacked 2.18 firmware by Liggy) will only burn them at 2,4x (the lowest speed)

I've gotten this info from disc identifier:
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Unique Disc Identifier : [DVD+R:MJC-003-000]
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Disc & Book Type : [DVD+R] - [Not Available]
Manufacturer Name : [MJC Pte. Ltd. Singapore ]
Manufacturer ID : [MJC]
Media Type ID : [003]
Product Revision : [Not Specified]
Blank Disc Capacity : [2,295,104 Sectors = 4,482.6MB = 4.38GB (4.70GB)]
Recording Speeds : [1x , 2.4x , 4x , 6x-8x]
---------------------------------------------------------------------------
[ DVD Identifier - http://DVD.Identifier.CDfreaks.com ]
---------------------------------------------------------------------------

and the media code seems to be very rare :/

anyone know of a (safe) way to burn these discs at a higher speed?
or a (safe ) firmware that will work for these discs?

I'd say post in the NEC forum and see if you and Liggy can help each other out -- I don't think many people have any of these discs at all, so you might have to be the guinea pig to try out some custom-built firmwares with various strategies for this media code. Once a good strategy is found, other people can benefit from it in a future Liggy firmware.

Hopefully you have access to a PI/PIF scanning drive so you can test which strategies work the best.

FWIW, the Benq 1620 can apparantly write this media at 12x.

Are you sure DMA hasn't been disabled?

uhm yeah, guess i'll try that.
don't usually test my discs though, so i'll need a little help on how to do that :P
dma is enabled.