The Sierra Compass 888 is indeed faster than the E1550 as it supports HSDPA (download) with 7.2 MBit/s (instead of 3.6 MBit/s) and HSUPA with up to 5.76 MBit/s. But on the German O2 network you will rarely get more than 2-3 MBit/s download, so the advantage in download of a device supporting higher HSDPA-categories is marginal. In contrast to the moderate increase of download bandwidth you will usually get > 1 MBit/s upload with an HSUPA-capable device (versus 0.384 MBit/s without HSUPA). In the end you would only benefit from the higher upload bandwidth, which becomes important when uploading files or sending eMails with large attachments and e.g. when video-calling through Skype (the higher your upload bandwidth, the better the video quality on the other end).
Before spending money for DC-unlocker consider buying a new HSPA-stick here in Germany. The Vodafone K3765 (unlocked, produced by Huawei and easily debrandable) is currently available
for ~ € 25 (incl. shipping) on eBay.de and O2 sell the
Huawei E1750 (easily unlockable and debrandable) for € 30 online and in their shops.