These are not exactly all "mods" as such, but things I've done to improve the frustratingly poor performance of my Navibot SR8855 and help make it slightly easier to live with.
1. Making it suck
If you look at your Navibot's dust bin, you've probably noticed how the frame of that filter half-blocks the suction channel. How Samsung didn't notice that themselves at the design stage we'll probably never know, but with a sharp hobby knife you can easily cut out the obstructing part of the frame to open it up (careful you don't cut into the filter itself - a thin lick of glue along that edge will ensure it's sealed). It certainly improves the suck on my 8855 - not brilliantly, but to some degree. Cleaning used/dirty filters in an ultrasonic cleaner improves the suck quite a bit too.
2. Making the battery run longer
Samsung warrant the Navibot battery for 6 months, and recommend replacing it after a year. There's nothing inherently wrong with the quality of the battery in my opinion just that the Navibot treats it very badly, and my 18-month old battery was down to a best run-time of just 26 minutes. Cycling it slowly 5-times improved that to a consistent run-time of 67 minutes - as good as a new one.
3. Straigtening out the curled-up side-brushes
No way am I buying new side brushes when there's nothing wrong with the old ones, other than they twizzle themselves up so tightly the Navibot starts jumping up and down like a nodding donkey. Piece of wood, 4 screws, leave them overnight, and they're as good as new by morning.
4. Virtual Wall/Lighthouse AA rechargable battery conversion
D batteries? They're kidding, right? Who still uses D batteries in this century? Do shops even sell them anymore? Well, not in my house... anything that needs batteries in my house will use rechargable AA or AAA cells. End of. The trouble is D batteries give 1.5 volts each, yet AA rechargables only give 1.2 volts, so you'll need 3 rechargable AA cells to make it work... and work brilliantly it does too. 3 AA cell battery holder and a clip easily available from Maplins or Radio Shack, it fits in the device perfectly, and with 2000mAh or better rechargable cells lasts just as long as typical D cells. I find "Hybrid" rechargable cells seem to work best since they retain peak charge much longer than normal rechargable cells.
2 x Karcher RC3000 (2005 & 2012), Trilobite ZA1 (2005), Navibot 8855 (2010)