I think you've answered your own question. Browning and adding veggies later will both help.
Browning doesn't "seal" the meat to keep flavor or juices in, but it does create a very nice flavor that's almost always associated with meat, caused by the Maillard reaction. I think browning could go a long way to resolving your flavor issue.
This is a little counter intuitive of cooking with a slow cooker, but you can start the meat with a bit of water (or wine, stock, other juices) and then only put the vegetables in much later in the process. Of course, then you have to tend your crock pot, which is often not the point. That would decrease the time that the vegetables had to affect the taste of your meat. You could even experiment with which ones to add later and which to add at the beginning. If it's just the carrots that are offensive, that may be the only thing you need to throw in later.
talon8 is absolutely right: the best way to go about this is to use a thermometer. However, it is still an interesting question how the expected cooking time varies with the weight of a roast.
It is such an interesting question that in 1961, SIAM Review published a scholarly article entitled "On Cooking a Roast". To be a bit more precise, the question that the authors answer is this: Suppose we have two roasts with exactly the same combination of identical tissues, and the same shape except that the one is a scaled (blown up) version of the other. Let's weigh both roasts and compute the ratio between the two weights; let's call that ratio r. Suppose furthermore that the roasts have the same starting temperature, and we will cook both until they have both reached some given (higher) temperature at their centre. What can we say about the times that both roasts need to cook for?
The answer is that under these assumptions, the bigger roast will need r^(2/3) times as long as the smaller one.
So, assuming each small roast (700 and 950 grams) has the same shape as the big one that the recipe writers used (let's say 1750 grams), for the 950 gram roast you'd expect to use (950 / 1750)^(2/3) = 0.665 or about two-thirds of the cooking time, and for the 700 gram one you'd expect to use (700 / 1750)^(2/3) = 0.543 or just over half of the cooking time. In particular, for the 950 gram roast, you should expect to roast it at 200C for 20 minutes, then turn down to 180 and roast for around 0.665 * 1750 / 450 * 12.5 = 32 minutes more. (1750 / 450 * 12.5 = 49 minutes is the average time they would recommend for the 1750 gram roast, and we're using that as the base for comparing our 950 gram roast against.) Then check with your thermometer!
Finally, I think for the resting time, you might as well use the original 30 minutes, because that's not (only) about getting the internal temperature to a certain level (although it does contribute to that - the heat will spread out through the meat), but also about things like the muscle fibers relaxing and the like, which are not covered by the article.
Best Answer
For the US crowd, silverside is the part of the round closest to sirloin, so it's a working cut and fairly lean.
In order to keep this juicy you'll need to bard it, in other words add fat. I'd do this by wrapping the whole thing up in streaky (US style) bacon and then sear it at high temperature to give it that crust before turning it down and continue the roasting at a lower temperature. So turn your oven up full-whack and let it get good and hot, then rub your roast with some salt, then into the oven for 20 minutes, then turn it down to 160 and roast it until it reaches 120F/50c. Remove and cover with foil, then let it rest for at least 30 minutes. If you don't let it rest it won't get tender at all.
In all honesty silverside isn't a cut I would roast, and I've tried. It just too lean and has too much connective tissue to be tender. It does make a good braise though, which is what I'd do with it.